E66.9: Obesity, unspecified

The ICD-10-CM code for obesity when the provider documents the diagnosis without specifying the class, the etiology or the severity.

Billable

FY2027

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At a glance

Code Family

E66: Overweight and obesity.

Condition

Obesity

Billable

Yes. E66.9 is valid for HIPAA-covered transactions.

HCC Mapping (v28)

None. E66.9 does not appear in the CMS-HCC v28 crosswalk. HCC 48 maps from E66.01, E66.2 and the adult BMI codes Z68.41 to Z68.45.

DRG Grouping

MS-DRG 640 or 641 in MDC 10 under the FY2027 MS-DRG v44 grouper when E66.9 is the principal diagnosis. E66.9 is on neither the CC list nor the MCC list for FY2027.

Family Specificity

Nine billable sibling codes structured around the etiology and the severity class of the obesity, with E66.9 as the residual code for obesity documented without either.

Which E66 code do I assign?

The E66 family splits on the etiology of the obesity and on the severity class the provider documents. Reserve E66.9 for obesity recorded without a class, an etiology or a severity.

When documentation says…

Code

Morbid obesity due to excess calories

E66.01

Morbid (severe) obesity due to excess calories. This code supports HCC 48. The provider must document morbid or severe obesity. When class 3 obesity is also documented, Guideline I.C.4.b.1 assigns only the class 3 code (E66.813).

Class 3 obesity, BMI 40 or greater

E66.813

Obesity, class 3. The provider must document the obesity class (Guideline I.C.4.b.1). The bundled CMS-HCC v28 crosswalk predates the class codes and lists none of them.

Obesity, class 1, BMI 30 to 34.9

E66.811

Obesity, class 1. Class 2 takes E66.812 instead. The provider must document the class (Guideline I.C.4.b.1).

Obesity due to excess calories, not morbid

E66.09

Other obesity due to excess calories. The etiology is documented but the obesity is not characterized as morbid or severe. This code carries no HCC.

Morbid obesity with alveolar hypoventilation

E66.2

Morbid (severe) obesity with alveolar hypoventilation, the code for Pickwickian syndrome. Obesity documented with alveolar hypoventilation supports HCC 48 capture.

Overweight

E66.3

Overweight. Overweight carries a separate code from obesity. Do not assign E66.9 when the provider documents overweight alone.

Obesity, no class or etiology stated

E66.9

Obesity, unspecified, the residual code for the family. Use it only when provider documentation does not permit a class-specific or etiology-specific code. Report the applicable Z68 code alongside it when BMI is documented.

Tabular notes

USE ADDITIONAL

Use additional code to identify body mass index (BMI), if known. The adult codes are Z68.18–Z68.45, for persons 20 years of age or older. Persons 2 to 19 years of age take a pediatric code (the Z68.5 codes). This instruction applies to E66.9. Guideline I.B.14 allows the BMI to come from another clinician, such as a dietitian. The obesity diagnosis must be documented by the patient's provider.

EXCLUDES2

Category E66 carries an Excludes2 note for adiposogenital dystrophy (E23.6), lipomatosis dolorosa [Dercum] (E88.2), lipomatosis NOS (E88.2) and Prader-Willi syndrome (Q87.11). An Excludes2 note means the excluded condition is not part of E66.9 and that a patient may have both. Both codes may be reported when both conditions are documented.

FY2027: EFFECTIVE 1 OCTOBER 2026

The FY2027 codes addenda make no addition, deletion or revision in category E66. E66.9 is unchanged in the FY2027 update. In the BMI codes, FY2027 replaced Z68.1 with Z68.18 for a BMI of 18.4 or less and Z68.19 for a BMI of 18.5 to 19.9. The obesity class codes sit under E66.81: E66.811 for class 1, E66.812 for class 2 and E66.813 for class 3. E66.811 and E66.813 first appear in FY2025, effective 1 October 2024, per the CMS MS-DRG Definitions Manuals, v41.0 (FY2024) and v42.0 (FY2025). Use E66.9 only when provider documentation does not permit assignment of a class-specific code.

Operational & financial impact

Why specificity matters beyond the chart: risk adjustment, DRG assignment and denial exposure all follow from which code is selected.

CMS-HCC v28 (PY2024+)

E66.9 does not map to an HCC under CMS-HCC v28.

E66.9 carries no risk adjustment weight.

HCC 48, Morbid Obesity, captures E66.01, E66.2 and the adult BMI codes Z68.41 to Z68.45 in the v28 crosswalk. The obesity class codes E66.811 to E66.813 postdate that crosswalk file. Their v28 mapping is not established there.

To reach HCC 48 through the obesity code itself, the provider must document morbid or severe obesity, which supports E66.01.

V24 TO V28 TRANSITION

E66.9 did not map to an HCC under CMS-HCC v24 either.

The move from v24 to v28 does not change the risk adjustment position of this code.

MS-DRG v44 (FY2027)

As a principal diagnosis, E66.9 groups to MS-DRG 640 or 641 in MDC 10 under the FY2027 MS-DRG v44 grouper. MS-DRG 640 is miscellaneous disorders of nutrition, metabolism, fluids and electrolytes with MCC. MS-DRG 641 is the same group without MCC.

E66.9 is on neither the CC list nor the MCC list in the FY2027 severity tables (CMS-1849-F, Tables 6I and 6J). As a secondary diagnosis it does not raise the DRG tier.

E66.01 is on neither list either. E66.2 and the adult BMI codes Z68.41 to Z68.45 are CCs.

Medicare and commercial payers

E66.9 is a valid diagnosis code for Medicare Part B claims.

Coverage for obesity-related services, including intensive behavioral therapy and bariatric surgery, is governed by National and Local Coverage Determinations carrying qualifying criteria that include BMI thresholds.

WHERE SPECIFICITY IS LOST

Assigning E66.9 when the provider documents morbid obesity drops the HCC 48 mapping that E66.01 carries.

Omitting the Z68 code when the BMI is recorded leaves the tabular Use Additional instruction unmet.

Leaving E66.9 in place when the provider documents an obesity class understates the record. The class takes a code from E66.811 to E66.813 (Guideline I.C.4.b.1).

Documentation checklist to support E66.9

Chart elements that support code selection and reduce specificity-driven denials.

A provider-documented diagnosis of obesity.

The obesity class where the provider can state it: class 1 directs to E66.811, class 2 to E66.812 and class 3 to E66.813.

The words morbid or severe obesity, which support E66.01, or class 3 obesity, which supports E66.813. When both are documented, Guideline I.C.4.b.1 assigns only the class 3 code.

The etiology where it is known. Excess calories directs to the E66.0- codes. A documented causative drug directs to E66.1, with the adverse effect of the drug coded additionally.

The BMI value, which another clinician such as a dietitian may document (Guideline I.B.14), to support the Z68 companion code.

Alveolar hypoventilation where it is documented alongside morbid obesity, which directs to E66.2.

Any coexisting Prader-Willi syndrome or adiposogenital dystrophy, which the Excludes2 note at E66 allows to be coded alongside E66.9.

For inpatient encounters, documentation at discharge supporting the highest level of specificity the physician record will bear.

FAQ

Is E66.9 a billable code?

Yes. E66.9 is valid for HIPAA-covered transactions. It is the unspecified member of category E66. Reach for a class-specific or etiology-specific code whenever the documentation supports one.

Does E66.9 map to an HCC under CMS-HCC v28?

No. E66.9 carries no HCC and no risk adjustment weight under CMS-HCC v28. It did not map to an HCC under v24 either. HCC 48, Morbid Obesity, maps from E66.01, E66.2 and the adult BMI codes Z68.41 to Z68.45. The obesity class codes postdate the bundled v28 crosswalk. Their mapping is not established there.

Can a coder assign E66.01 from a BMI of 40 without provider documentation of morbid obesity?

No. Guideline I.B.14 allows the BMI to come from another clinician. The associated diagnosis must be documented by the patient's provider. E66.01 requires provider documentation of morbid or severe obesity. Documented class 3 obesity takes E66.813. Guideline I.C.4.b.1 assigns only the class 3 code when both are documented. The BMI itself is reported with a Z68 code.

Should E66.9 carry a Z68 BMI code?

Report a Z68 code when the BMI is documented in the record. The tabular instruction reads if known. Guideline I.B.14 allows the BMI to be taken from another clinician's documentation, such as a dietitian's. Omit the Z68 code when no BMI is recorded. BMI codes are not assigned during pregnancy.

What changed for obesity coding in FY2027?

Category E66 is unchanged in the FY2027 update, effective 1 October 2026. E66.9 remains valid for documentation that does not specify the class. The class-specific codes E66.811 and E66.813 first appear in FY2025, effective 1 October 2024, per the CMS MS-DRG Definitions Manuals, v41.0 (FY2024) and v42.0 (FY2025). In the BMI codes, FY2027 replaced Z68.1 with Z68.18 for a BMI of 18.4 or less and Z68.19 for a BMI of 18.5 to 19.9.

Can E66.9 be the principal diagnosis?

Yes. E66.9 can be the principal diagnosis when obesity is the reason for the encounter, such as a bariatric consultation. Otherwise E66.9 is reported as a secondary diagnosis. Guideline I.B.14 reports BMI codes only as secondary diagnoses.

How does E66.9 differ between the outpatient and inpatient settings?

Outpatient: report E66.9 when the provider documents obesity without a class. Add the applicable Z68 code when BMI is documented. Inpatient: E66.9 is on neither the CC list nor the MCC list for FY2027. Code to the highest level of specificity the physician documentation supports at discharge.

Which codes cannot be reported alongside E66.9?

Category E66 carries no Excludes1 note. Its Excludes2 note lists adiposogenital dystrophy (E23.6), lipomatosis dolorosa [Dercum] (E88.2), lipomatosis NOS (E88.2) and Prader-Willi syndrome (Q87.11). An Excludes2 note allows both codes when the patient has both conditions. Chapter 4 carries an Excludes1 note for transitory endocrine and metabolic disorders specific to newborn (P70-P74).

Related codes

Codes in the same family as E66.9. Only codes with a published page render as links.

Sibling codes in the E66 family

E66.01: Morbid (severe) obesity due to excess calories.
E66.09: Other obesity due to excess calories.
E66.1: Drug-induced obesity.
E66.2: Morbid (severe) obesity with alveolar hypoventilation.
E66.3: Overweight.
E66.811: Obesity, class 1.
E66.812: Obesity, class 2.
E66.813: Obesity, class 3.
E66.89: Other obesity not elsewhere classified.

Additional codes to consider

Codes outside the E66 family considered alongside E66.9.

Companion: code alongside when documented

Z68.18–Z68.45: Body mass index [BMI], adult, called for by the Use additional note at category E66 for persons 20 years of age or older.
Z68.5-: Body mass index [BMI] pediatric, a subcategory that requires further characters, called for by the same note for persons 2 to 19 years of age.

Consider instead

E66.01: Morbid (severe) obesity due to excess calories, where the provider documents morbid or severe obesity.
E66.813: Obesity, class 3, where the provider documents the class.
E66.3: Overweight, where the provider documents overweight rather than obesity.
E23.6: Other disorders of pituitary gland, the code for adiposogenital dystrophy.
E88.2: Lipomatosis, not elsewhere classified.
Q87.11: Prader-Willi syndrome.

Sources

All claims on this page derive from government and professional-standards sources.

National Center for Health Statistics. (2026). International classification of diseases, tenth revision, clinical modification (ICD-10-CM) FY2027 files. https://www.cdc.gov/nchs/icd/icd-10-cm/files.html
Centers for Medicare & Medicaid Services and National Center for Health Statistics. (2026). ICD-10-CM official guidelines for coding and reporting FY2027. https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Publications/ICD10CM/2027/ICD-10-CM-October-1-2026-FY27-Guidelines.pdf
Centers for Medicare & Medicaid Services. (2025). 2026 model software/ICD-10 mappings. https://www.cms.gov/medicare/payment/medicare-advantage-rates-statistics/risk-adjustment/2026-model-software-icd-10-mappings
Centers for Medicare & Medicaid Services. (2026). MS-DRG classifications and software, version 44. https://www.cms.gov/medicare/payment/prospective-payment-systems/acute-inpatient-pps/ms-drg-classifications-and-software