Non-insulin-dependent diabetes mellitus (NIDDM) with onset below 35 years of age was studies in 43 Indian and 9 Black patients. NIDDM was diagnosed in 10.0% and 1.6% of diabetes in the respective racial groups. The mean age of the patients was 35.4 years and the mean duration of diabetes 7.5 years. Nearly 50% of patients were significantly obese, and more than 50% had a positive family history of diabetes. In all patients the diabetes was symptomatic at presentation, although frequently insidious in onset. In all cases the symptoms could be controlled without insulin therapy. Despite persistence significant hyperglycaemia in several patients, ketosis did not occur at any stage. Complications were detected in 40% of the Indian and 22% of the Black patients, but were unrelated to the duration of diabetes or severity of hyperglycaemia. Basal insulin levels and postglucose insulinaemic responses were lower in the Black than the Indian diabetes.