Plasma C-peptide levels were measured in classical insulin-dependent diabetics (group I) and in patients who had become insulin-dependent after a mean 12 years of non-insulin dependent diabetes (group II). All had been under insulin therapy for no more than 2 years. Metabolic control, as assessed by blood glucose and glycosylated A1C haemoglobin levels, was equally poor in both groups. The doses of insulin required were almost identical, though slightly higher in group II, while C-peptide levels were twice as high in group II patients as in group I patients. These findings suggest that patients with initially non-insulin dependent diabetes are more resistant to insulin than classical insulin-dependent diabetics. This low sensitivity to insulin might be due to age (which was more advanced in our group II patients) or might indicate that these patients still retain a degree of insulin-resistance that is characteristic of non-insulin dependent diabetes.