Glucose tolerance was assessed, according to WHO diagnostic criteria, in 506 consecutive African patients admitted with sputum-positive pulmonary tuberculosis to the tuberculosis wards of Muhimbili Medical Centre, Dar es Salaam. Nine (1.8%) patients were known to have diabetes. Following a 75 g oral glucose tolerance test (OGTT) diabetes was diagnosed in a further 25 (4.9%) patients giving an overall crude diabetes prevalence rate of 6.7%. Impaired glucose tolerance (IGT) was present in 82 (16.2%) subjects. A repeat OGTT was carried out in the 25 patients found to have diabetic values after the first test. Eight (28%) of the 25 patients reverted to normal glucose tolerance after the second test, 6 (24%) to IGT, and 11 (48%) remained with blood glucose values in the diabetic range, giving a crude diabetes prevalence rate of 4%. In a survey of glucose tolerance, using similar methodology, in 693 members of an urban community in Dar es Salaam the prevalence rates of diabetes and IGT were 0.9% and 8.8% respectively. Diabetes was therefore at least four times as common in the tuberculosis patients (p less than 0.001), and IGT twice as frequent (p less than 0.0001). This study confirms the relationship between diabetes and tuberculosis. Those caring for patients with tuberculosis should be aware of the increased prevalence of diabetes in their patients, since failure to diagnose the problem may adversely affect prognosis.