Serum lipids and lipoproteins were measured in 277 non-insulin-dependent diabetics (NIDDs) and in 124 non-diabetic control subjects (65 males, 59 females), aged 45-64 years. Altogether 88 of the diabetics were treated with diet (48 males, 40 females), 134 with oral drugs (56 males and 49 females treated with sulphonylureas, 14 males and 15 females treated with a combination therapy of sulphonylurea drug and metformin) and 55 with insulin (17 males, 38 females). The postglucagon C-peptide concentration in insulin-treated diabetics exceeded 0.60 nmol/l. The diabetics had lower levels of HDL and HDL2 cholesterol and higher levels of total and VLDL triglycerides than non-diabetic control subjects irrespective of the mode of treatment. The HDL2 subfraction seemed to be alone responsible for the decrease of HDL cholesterol. In the whole group of diabetics body mass index had a significant negative correlation to HDL cholesterol and a positive correlation to total triglyceride concentration in both sexes but plasma glucose failed to show any consistent association to HDL cholesterol concentration. The difference in HDL cholesterol between diabetics and non-diabetics persisted after adjustment for age, physical activity, alcohol intake and body mass index. In conclusion, the dyslipoproteinaemia in non-insulin-dependent diabetes is principally characterized by decreased HDL and HDL2 cholesterol concentrations and by increased total and VLDL triglycerides. These manifestations of dyslipoproteinaemia are little influenced by the degree of glycaemia and obesity.