OBJECTIVE: When presenting with diabetic ketoacidosis (DKA), lean and obese patients differ in their subsequent clinical course. Although lean patients tend to remain insulin dependent, most obese patients recover endogenous insulin secretion and discontinue insulin therapy. The aim of this study was to determine whether obese African-American patients with DKA could be determined to have type 1 or type 2 diabetes based on insulin secretion or the presence of immunological and genetic markers. RESEARCH DESIGN AND METHODS: This was a prospective study that analyzed the clinical characteristics, insulin secretion indices, immunological markers (islet cell, GAD, ICA512, and insulin autoantibodies), and HLA susceptibility genes (DR/DQ) in 131 patients with DKA (77 obese and 54 lean), 51 obese patients with hyperglycemia but no DKA, and 25 nondiabetic subjects. All subjects were African-American. Beta-cell function was evaluated by the C-peptide response to glucagon (1 mg i.v.) within 48 h of resolution of DKA or hyperglycemia. RESULTS: The acute C-peptide response was lower in obese DKA patients (1.0+/-0.1 ng/ml) than in obese patients with hyperglycemia (1.7+/-0.2 ng/ml, P < 0.01), but was higher than that in lean DKA patients (0.2+/-0.1 ng/ml, both P < 0.01). The overall prevalence of autoantibodies in obese subjects with DKA (17%) and obese subjects with hyperglycemia (16%) was lower than that in lean subjects with DKA (65%, P < 0.01). Obese patients with hyperglycemia and positive autoantibodies had lower rates of insulin secretion than those without antibodies. Regardless of body weight, all DKA patients with GAD autoantibodies carried the DQB1*0201 allele. However, there were no significant differences in HLA distribution between the three patient groups. CONCLUSIONS: Our results indicate that most obese African-American patients with DKA have type 2 diabetes characterized by higher insulin secretion, the absence of autoimmune markers, and a lack of HLA genetic association. In contrast, most lean African-American patients with DKA have metabolic and immunological features of type 1 diabetes. At presentation, assessment of beta-cell function and determination of autoimmune markers allow for correct classification of diabetes in African-Americans with hyperglycemic crises.