The concentration of cholesterol in serum high density lipoproteins (HDL) and their subfractions (HDL2 and HDL3) was determined in men and women with type I diabetes treated with insulin and in non-diabetic men and women matched for age, weight and serum cholesterol, but not for triglyceride-rich lipoproteins which were higher in the diabetic patients. In none of the patients was the diabetes badly controlled as assessed by measurement of the percentage of haemoglobin in the glycosylated state (HbA1). Regardless of their diabetic control, significantly increased serum HDL3 cholesterol concentration was found in male diabetic patients compared to non-diabetic controls, and in both male and female diabetics the ratio of HDL2 to HDL3 cholesterol was significantly reduced. In patients with well-controlled diabetes (HbA1 less than or equal to 9%) serum HDL3 cholesterol was significantly increased in both men and women, whereas in those with less well-controlled diabetes (HbA greater than 9%) it was similar to that of non-diabetic controls. In only one group of patients, the less well controlled men, was there a significant reduction in serum HDL2 cholesterol. On the basis of current theory, the results of this study would support the view that HDL3 does not contribute to the inverse relationship between total serum HDL and the risk of ischaemic heart disease.