OBJECTIVES: To examine trends of prevalence and treatment patterns of diabetes mellitus (DM) in adults and to determine the relationship between DM and obesity. METHODS: This study used Third National Health and Nutrition Examination Survey (NHANES III for 1988-1994) and NHANES 1999-2002, representing a national sample of the non-institutionalized civilian US population. Adult DM patients were identified if they were ³20 years old, and were previously diagnosed with DM by a physician or currently using DM therapy (hypoglycemic agent/insulin). These patients were classified as obese if body mass index ³30. Nationally representative prevalence was estimated using sample weights. Data were analyzed using SAS and SUDAAN statistical softwares to adjust for the complex sample design. RESULTS: The age-adjusted prevalence of DM was 6.8% in 1999-2002 compared to 5.4% in 1988-1994 (p<0.001). Approximately 51.6% of DM patients were obese in 1999-2002 (44.1% in 1988-1994). During 1999-2002, DM was more prevalent in patients aged 65-74 (16.31%) and was increased most rapidly in this age group (4.5% increase;p<0.05). DM was most prevalent in Whites (61%;12.7% decrease from NHANES-III;p<0.05), followed by Blacks (16%;1% increase), and Hispanics (16%;10% increase;p<0.05). More obese DM patients were treated with medication than non-obese DM patients (84%vs.80%). During study period, DM patients who received treatment significantly increased by 8% (p<0.01;6% in obese), but 62% of these patients' A1c was still uncontrolled. DM patients often have comorbidities of hypertension (67%;4% increase), followed by hyperlipidemia (48%;12% increase), and retinopathy (27%;11% increase) and these comorbidities are more prevalent in obese DM patients. CONCLUSIONS: DM has become very rapidly prevalent in US adults, with the greatest increase in Hispanics. Approximately half of DM patients did not have their A1C level controlled and one-fifth still did not receive treatment. DM patients who were obese were less likely to control their A1c compared to overall DM patients.