The prevalence of diabetes and impaired glucose tolerance has been determined in an Asian Muslim community in Dar-es-Salaam, Tanzania. Two-h oral glucose (75 g) tolerance tests were performed on 1049 subjects over 14 years old, who were fasting, from a random sample of families. The overall age and sex-adjusted prevalence of diabetes was 7.1% (4.4% known, 2.7% previously undiagnosed) with a steady increase from 0.8% at 15-24 years and 3.0% at 25-34 years, to 24.9% for 65 years and over. Impaired glucose tolerance (IGT) rates ranged from 11.4% (15-24 years) to 22.3% (over 64 years). The overall age-adjusted prevalence of IGT was 21.5%. The mean body indices (BMIs) were 24.3 and 26.4 for males and females, respectively, but age-adjusted diabetes rates were similar in the two sexes (7.0% and 7.6%, respectively). Diabetes and IGT were commoner in those with BMI greater than 25 only in the older age groups. Diabetes and IGT were commoner in those with a family history of diabetes. Increasing parity was also associated with a higher diabetes prevalence. Diabetes and IGT are thus common in Asians in Tanzania, in contrast to the indigenous community. Rates are indeed higher than in most other immigrant Asian communities.