CONTEXT: Morbid obesity (body mass index (BMI) Z40 kg/m 2 ) is associated with substantially increased morbidity and mortality from chronic health conditions and with poorer health-related quality of life; however, less is known about the impact of morbid obesity on healthcare expenditures. OBJECTIVE: To examine the impact of morbid obesity on healthcare expenditures using a nationally representative sample of US adults. DESIGN, SETTING, AND PARTICIPANTS: We performed a cross-sectional analysis of 16 262 adults from the 2000 Medical Expenditure Panel Survey, a nationally representative survey of the noninstitutionalized civilian population of the United States. Per capita healthcare expenditures were calculated for National Institutes of Health BMI categories, based on self-reported height and weight, using a two-part, multivariable model adjusted for age, gender, race, income, education level, type of health insurance, marital status, and smoking status. MAIN OUTCOME MEASURES: Odds of incurring any healthcare expenditure and per capita healthcare expenditures associated with morbid obesity in 2000. RESULTS: When compared with normal-weight adults, the odds of incurring any healthcare expenditure in 2000 were two-fold greater among adults with morbid obesity. Per capita healthcare expenditures for morbidly obese adults were 81% (95% confidence interval (CI): 48-121%) greater than normal-weight adults, 65% (95% CI: 37-110%) greater than overweight adults, and 47% (95% CI: 11-96%) greater than adults with class I obesity. Excess costs among morbidly obese adults resulted from greater expenditures for office-based visits, outpatient hospital care, in-patient care, and prescription drugs. Aggregate US healthcare expenditures associated with excess body weight among morbidly obese US adults exceeded $11 billion in 2000. CONCLUSIONS: The economic burden of morbid obesity among US adults is substantial. Further research is needed to identify interventions to reduce the incidence and prevalence of morbid obesity and improve the health and economic outcomes of morbidly obese adults.