Insulin-dependent diabetes mellitus (IDDM) with onset below 35 years of age was studied in 52 Black and 38 Indian patients. IDDM accounted for approximately 10.4% and 1.1% of diabetes in the respective racial groups. The mean age and body weight in the Black and Indian diabetics were 27.6 years and 24.7 years, and 60.2 kg and 54.7 kg, respectively. The duration of diabetes in the majority of Blacks was 1-4 years, and that in Indians 5-9 years. The initial presentation in more than 80% of the patients was acute, with severe keto-acidosis in 15%. A positive family history of diabetes was obtained in more than 50% of Indians and in less than 6% of Blacks. Complications were present in 33% of Indian patients and were related to the duration of illness and dose of insulin required. Basal growth hormone, cortisol, cholesterol and triglyceride concentrations in serum were higher in Indians than in Blacks, but the differences were not significant. The disease was unrelated to excess alcohol intake or to pancreatic calcification.