OBJECTIVES: To estimate treatment costs attributable to overweight or obesity in diabetic patients. METHODS: Data from 23,825 adults in the 2004 Medical Expenditure Panel Survey (MEPS), a nationally representative sample of the U.S. non-institutionalized population, were examined for this cross-sectional analysis. Diabetic patients were identified if they reported having diabetes in the medical conditions file of MEPS. Based on their BMI, patients were further classified as normal (body mass index (BMI 18.5 to <25), overweight (BMI 25 to <30), or obese (BMI³30). Treatment costs associated with diabetes mellitus included costs for treating kidney and heart disease, stroke, eye, skin, and foot complications, and neuropathy. The costs for overweight or obese patients were predicted from estimated parameters in patients with normal weight using a generalized linear model with log link function, with adjustment for patients' demographic characteristics, BMI, services used, and co-morbidities. Attributable costs were estimated by the difference between predicted costs and observed costs. All costs were converted to 2005 U.S. dollars using appropriate price indices. Data were analyzed using SAS and SUDAAN statistical software to adjust for the complex sample design. RESULTS: The age-adjusted prevalence of diabetes in the U.S. population was 6.26% (15.3 million people). More than 80% of diabetic patients were overweight (30.4%) or obese (51.4%). Among obese patients, the medical costs associated with diabetes were higher in women ($6263) than men ($4505), as well as in white ($6209) than black ($4819) or Hispanic ($2421) patients. Overall, compared to patients with normal weight, predicted costs attributable to overweight were $709 (95% CI:$237-$1181) and obesity, $2301 (95% CI:$748-$3854). CONCLUSION: Overweight and obesity were prevalent in diabetic patients that significantly increased economic burden. Patient education to emphasize the importance of weight control is strongly recommended to reduce treatment costs attributable to diabetic complications.