Analysis of the age of onset of diabetes amongst insulin-treated patients in a large African diabetic clinic revealed a bimodal type of distribution, 23 per cent having an age of onset before 30 years and 77 per cent with onset at greater than or equal to 30 years of age. All 66 of the young insulin-treated group (21.7 +/- 4.8 years (mean +/- 1 SD)), and a random selection of 50 older insulin-treated patients (49.7 +/- 10 years), were studied. The older group were better controlled (HbA1 8.4 +/- 1.7 per cent vs. 10.8 +/- 2.6 per cent, p less than 0.001), on lower doses of insulin (49 +/- 23 vs. 71 +/- 23 u/day, p less than 0.001) and had higher body mass index (26.0 +/- 5.6 vs. 21.8 +/- 3.5, p less than 0.001). Serum C-peptide (0.24 +/- 0.15 vs. 0.07 +/- 0.10 nmol/l, p less than 0.0001), and C-peptide/glucose ratio (2.57 +/- 2.65 vs. 0.56 +/- 0.98 nmol/mmol x 10(2), p less than 0.001) were very significantly higher in older patients. Patients with later onset disease thus had better preservation of pancreatic function, higher body mass index and better glycaemic control on lower doses of insulin. These features suggest that older insulin-treated patients could in fact be 'Type 2' or non-insulin dependent patients, and the condition may be controllable with diet and/or oral hypoglycaemic agents, at least in some.