In 122 diabetic patients there was a significant inverse correlation between plasma-high-density-lipoprotein (H.D.L.)-cholesterol concentrations and glycosylated hæmoglobin concentrations. In an insulin-treated subgroup improvement in diabetic control, confirmed by a decrease in glycosylated hæmoglobin concentrations, was accompanied by a significant rise in plasma-H.D.L.-cholesterol concentration. This result suggests that improved diabetic control may diminish the risk of premature ischæmic heart-disease. When three groups of patients on different treatments were matched for age, sex, weight, and diabetic control, diabetic patients receiving sulphonylureas had significantly lower plasma-H.D.L.-cholesterol concentrations than those receiving insulin or on diet alone, or non-diabetic controls. This association suggests that sulphonylurea administration may constitute a positive risk factor for ischæmic heart-disease in diabetic patients. © 1978.