The study assesses whether diabetes has an effect on serum high density lipoprotein (HDL) cholesterol concentration independent of other factors known to influence serum HDL cholesterol concentration. Concentrations of serum HDL cholesterol, serum cholesterol, serum M particles and blood sugar, and the proportion of haemoglobin in the form of HbA1 were measured in a diabetic and a non-diabetic polulation. Relative body weight, alcohol and cigarette consumption, age, a clinical estimate of diabetic control and duration of disbetes were also recorded. The diabetic patients were divided into those in whom insulin treatment was clinically indicated and those in whom it was not. Serum HDL cholesterol concentration was significantly higher in diabetic men treated with insulin than in normal men and also higher in diabetic women treated with insulin than in normal women. In the diabetic men and women not treated with insulin, serum HDL cholesterol concentration was not significantly different from normal. There were differences between the diabetic and non-diabetic populations in terms of factors known to influence serum HDL cholesterol and also in the degree of correlation between these and the serum HDL cholesterol. Multivariate analysis revealed that in both male and female diabetic patients treated with insulin, diabetes was a highly significant influence on serum HDL cholesterol concentration, but in the non-insulin-treated diabetic patients the influence was absent in women and only marginal in men. The proportion of HbA1 influenced serum HDL cholesterol concentration negatively in insulin-treated diabetes but not in diabetes treated without insulin.