We studied sex differences in the serum lipid abnormalities associated with diabetes mellitus in 111 patients with insulin-dependent diabetes and 270 patients with non-insulin-dependent diabetes, who were compared with 586 nondiabetic controls. Relative to control levels, the increases in triglycerides were 17 to 34 mg per deciliter greater in diabetic women than in diabetic men. The median low-density-lipoprotein cholesterol concentration in non-insulin-dependent diabetics was 1 to 4 mg per deciliter lower than the control level in women and 16 to 22 mg per deciliter lower in men, and was 30 mg per deciliter higher than control in insulin-dependent diabetic women and similar to control in insulin-dependent diabetic men. The decrease in median high-density-lipoprotein cholesterol in non-insulin-dependent diabetics was 2 to 7 mg per deciliter greater in women than in men, and the increase in high-density-lipoprotein cholesterol in insulin-dependent diabetics was 3 mg per deciliter less in women than in men. We conclude that diabetes has a greater adverse effect on triglyceride and lipoprotein cholesterol concentrations in diabetic women than in diabetic men, and that this may explain the greater increase in risk of arteriosclerosis in diabetic women.