E66.01 — Morbid (severe) obesity due to excess calories

Clinical Definition

Morbid (severe) obesity due to excess calories is a chronic metabolic disease characterized by excessive body fat accumulation resulting from a sustained positive energy balance — caloric intake exceeding expenditure over a prolonged period. ICD-10-CM code E66.01 designates the most clinically severe form of obesity when the etiology is attributed to excess caloric consumption rather than drug-induced causes (E66.1), genetic syndromes, or endocrine disorders. Morbid obesity carries significantly elevated risks of type 2 diabetes, hypertension, obstructive sleep apnea, cardiovascular disease, nonalcoholic fatty liver disease, osteoarthritis, and premature mortality.

The clinical threshold for morbid obesity is defined by body mass index (BMI). A patient qualifies with a BMI of 40.0 kg/m2 or greater (Class III obesity), or a BMI of 35.0-39.9 kg/m2 (Class II obesity) when accompanied by at least one obesity-related comorbidity such as type 2 diabetes, hypertension, or obstructive sleep apnea. NIH and CMS guidelines both recognize these dual criteria. For pediatric patients, severe obesity is defined as a BMI at or above 120% of the 95th percentile for age and sex, or a BMI of 35 kg/m2 or greater, whichever is lower.

ICD-10-CM coding guidelines require that E66.01 be paired with a BMI Z-code from category Z68. Applicable codes include Z68.41 (BMI 40.0-44.9), Z68.42 (BMI 45.0-49.9), Z68.43 (BMI 50.0-59.9), Z68.44 (BMI 60.0-69.9), and Z68.45 (BMI 70 or greater). The BMI Z-code may be documented by any qualified healthcare professional (nurse, dietitian), but the morbid obesity diagnosis itself must be documented by the responsible provider. Failure to pair E66.01 with the corresponding Z68 code is a common audit finding and may result in claim denials.

When to Use E66.01

• ✓The provider documents morbid obesity, severe obesity, or Class III obesity in a patient with a BMI of 40.0 kg/m2 or greater, and the obesity is attributed to excess caloric intake rather than medication, genetic, or endocrine causes.
• ✓The provider documents severe obesity in a patient with a BMI of 35.0-39.9 kg/m2 who has one or more obesity-related comorbidities (type 2 diabetes, hypertension, obstructive sleep apnea, osteoarthritis), and the documentation attributes the obesity to excess calories.
• ✓A patient is being evaluated for bariatric surgery and the provider documents morbid obesity due to excess calories as the qualifying diagnosis for the surgical intervention.
• ✓The provider documents morbid obesity due to excess calories as a chronic condition on the problem list during an annual wellness visit, chronic disease management encounter, or medical weight management visit.
• ✓A patient is admitted to the hospital and the provider documents morbid obesity due to excess calories as a secondary diagnosis that affects the plan of care, nursing requirements, or complicates a surgical procedure.
• ✓The provider initiates or continues pharmacotherapy for weight management (e.g., GLP-1 receptor agonists, phentermine-topiramate) and documents morbid obesity due to excess calories as the indication for treatment.

Common Coding Mistakes

Related & Differential Codes

Documentation Requirements

• 1Diagnosis specificity: the provider must document 'morbid obesity,' 'severe obesity,' or 'Class III obesity' to support E66.01. Generic terms such as 'obesity' or 'obese' without a severity qualifier default to E66.09 and do not justify E66.01.
• 2BMI value: a current BMI must be documented in the medical record. For adults, a BMI of 40.0 or greater supports E66.01 independently; a BMI of 35.0-39.9 supports E66.01 only when obesity-related comorbidities are also documented.
• 3BMI Z-code pairing: the corresponding Z68 code must be assigned alongside E66.01. The BMI may be documented by any qualified clinical staff member (physician, nurse, dietitian, medical assistant), but the Z-code must reflect the BMI recorded during the encounter.
• 4Etiology attribution: the documentation should indicate that the obesity is due to excess caloric intake. If the obesity is secondary to medication use, endocrine disorders, or genetic conditions, a different E66 subcategory or alternate code should be used.
• 5Comorbidity documentation: when the BMI is 35.0-39.9, at least one obesity-related comorbidity (type 2 diabetes, hypertension, obstructive sleep apnea, osteoarthritis, etc.) must be documented and linked to the obesity to support the 'morbid' designation.
• 6Clinical significance: for inpatient encounters, document how the morbid obesity affects the plan of care — such as requiring specialized equipment, modified surgical approach, increased nursing care, or altered drug dosing — to support its reporting as a secondary diagnosis.
• 7Longitudinal tracking: for ongoing weight management, document the current BMI at each encounter along with the trend (weight change since last visit), interventions in place (dietary counseling, pharmacotherapy, bariatric surgery evaluation), and treatment response.
• 8Pediatric considerations: for patients under 21, document BMI percentile for age and sex rather than raw BMI category. Severe obesity in pediatric patients is defined as BMI at or above 120% of the 95th percentile or BMI of 35 or greater, whichever is lower.

Reimbursement & Billing Notes

ICD-10-CM code E66.01 is a valid, billable diagnosis code accepted by Medicare, Medicaid, and commercial payers. It maps to CMS-HCC category 48 (Morbid Obesity) for Medicare Advantage risk adjustment, making it a significant code for value-based care and capitated payment models. Accurate annual documentation and coding of E66.01 directly affects risk-adjusted revenue. The code supports medical necessity for a broad range of services including intensive behavioral therapy for obesity (covered by Medicare with no cost-sharing), medical nutrition therapy, pharmacotherapy for weight management, and bariatric surgery when additional clinical criteria are met. Most payers require prior authorization for bariatric procedures and will validate that E66.01 is supported by documented BMI values, comorbidities, and evidence of failed conservative management.

E66.01 must be paired with a BMI Z-code from category Z68 for claims to process without issue. CMS and many commercial payers have implemented edits that flag E66.01 claims lacking a Z68 code, which can result in claim rejections, requests for additional documentation, or post-payment audit recoupment. The BMI documentation does not need to come from the billing provider — it can be recorded by nursing staff, dietitians, or medical assistants — but it must be present in the encounter record. For risk adjustment submissions (RAF/HCC), E66.01 must be reported at least once annually during a face-to-face encounter with an eligible provider to be captured in the risk score. Providers should avoid carrying forward E66.01 from prior years without re-documenting and re-assessing the diagnosis at the current encounter.

Frequently Asked Questions