A population-based survey of the prevalence of insulin-treated diabetes mellitus in the Dundee area showed a crude prevalence of 0.34%, but analysis by interval between diagnosis of diabetes and start of insulin shows that around a third of patients started insulin therapy more than 1 month after diagnosis. This suggests that the prevalence of insulin-dependent diabetes is around 0.2% of the population, with the other patients being those with non-insulin-dependent diabetes who need insulin for metabolic control, though there is an intermediate zone where classification is difficult. There is a male excess. The proportion of true insulin-dependent diabetes falls with age of onset, but around 50% of such patients have an onset over the age of 30 years.