Type I (insulin-dependent) diabetes mellitus (IDDM) appears to be rare in indigenous African populations, but little detailed information has been published. We have therefore analysed the age of onset of diabetes in 176 African patients with IDDM (age of onset < 35 years), and in 149 consecutive white patients of European extraction for comparison. In the Africans the peak age of onset occurred at 22-23 years (median 22 years) with an earlier peak from 14 to 17 years. In the Whites, the peak was found at 12-13 years (median 12 years). In only 7% of Africans did diabetes start under the age of 12 years. There was a slight female preponderance in the Africans, especially at the ages of greatest incident (20-25 years)-F:M 1.55:1. When patients with duration of diabetes less than 5 years only were analysed (the period during which early mortality among African patients was greatly reduced) the patterns of age distribution were similar to the total respective groups. A peak incidence in the winter months was noted.