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OBJECTIVE: The Veterans Affairs Cooperative Study in Type 2 Diabetes Mellitus was conducted in NIDDM patients to determine if a significant difference in HbA1c could be achieved between groups receiving standard and intensive treatment. We observed differences in the response to exogenous insulin between African-Americans and other intensively treated patients. Therefore, we assessed the variations of response and correlated factors that might explain such differences. RESEARCH DESIGN AND METHODS: One hundred fifty-three men aged 40-69 years with NIDDM for < or = 15 years were randomized to either the standard therapy (n = 78) or the intensive therapy (n = 75) arm. Of the 75 patients in the intensive therapy group, 57 completed the study on insulin therapy alone. Of these, 18 were African-Americans and 39 were non-African-Americans. We conducted an analysis of the data collected to determine differences in baseline characteristics, glycemic response, insulin requirement, body weight, exercise, and basal C-peptide level, factors that may explain a difference in response to insulin therapy. RESULTS: Glycemic control improved in all patients with intensive insulin therapy. African-Americans achieved a greater improvement in HbA1c compared with non-African-Americans with a similar increment in insulin. This difference could not be explained by differences in body weight, activity, concomitant use of other medicines, or insulin-secretory capacity of the pancreas. CONCLUSIONS: We conclude that ethnic differences may exist in the response to insulin therapy. A knowledge of such differences may aid in achieving good glycemic control, especially since minorities have a greater prevalence of and burden from the microvascular complications of diabetes.