The prevalence of diabetes mellitus in 3,145 Tanzanian Africans living in three different areas of the country was studied. Fasting capillary blood glucose concentrations were measured by Dextrometer and if the levels were ⩾ 5.5 mmol/l an oral glucose tolerance test was performed. When using the 1980 WHO criteria  for diagnosis, the overall diabetic prevalence was 0.7%. Prevalence increased with age, and in the population ⩾ 20 years of age it was 1.6%. In a rural area inhabitated by the Haya tribe, the prevalence in the population aged ⩾ 20 years was 2.5%, and in a similar area populated by the Sukuma tribe it was 0.5%. In the urban area of Mwanza town it was 1.9%. Obesity was seen in 3.7% of the population and in 9.1% of the diabetic subjects. Sixty-eight percent of the diabetic patients were female compared with 53% in the general population. None of the diabetic patients discovered had any symptoms. Thus, the overall prevalence of diabetes in Tanzania is rather low but shows geographical variability and is strongly associated with age.