To study the clinical characteristics of newly diagnosed diabetic patients in tropical Africa. Prospective study of all newly diagnosed diabetic patients registered at a major urban hospital between 1 June 1981 and 31 May 1987. Muhimbili Medical Centre, Dar es Salaam, Tanzania. 1250 Patients: 874 men, 376 women. 272 (21.8%) Patients had diabetes requiring insulin, 825 (66.0%) had diabetes not requiring insulin, and 153 (12.2%) had diabetes of uncertain type. Most patients (1103, 88.2%) presented with the classic symptoms of diabetes. The peak time of presentation of diabetic patients requiring insulin was at age 15 to 19 years. Male manual workers and peasant farmers with diabetes not requiring insulin presented at a significantly older age and had a lower body mass index than sedentary office workers. Forty six (18.1%) of the patients requiring insulin diabetes and 111 (14.4%) not requiring insulin had first degree relative with diabetes. Twenty seven per cent of patients were underweight (body mass index less than 20 kg/m2) and 14.6% were obese (body mass index greater than 30 kg/m2). Hypertension was diagnosed in 211 (26.7%) of 791 patients not requiring insulin. Nine (3.3%) of those requiring insulin may have had the protein deficient type of diabetes related to malnutrition. The fibrocalculous variety of diabetes related to malnutrition was not observed. Newly presenting diabetic patients in Tanzania with diabetes requiring insulin are older at presentation than those in Britain; most diabetic patients present with diabetes not requiring insulin and a smaller proportion of Tanzanian patients are obese. Most have a lower socioeconomic state than diabetic patients in Britain. There are often delays in diagnosis in Tanzania, and there is a higher incidence of death shortly after presentation.