Diurnal profiles of total and lipoprotein cholesterol and triglycerides were measured in 11 insulin-dependent diabetic subjects without retinopathy, 10 with background and 10 with proliferative retinopathy. The groups were closely matched for age and duration of diabetes. Total cholesterol levels were higher in patients with proliferative (5.6 +/- 0.5 mmol/l) than background (5.1 +/- 0.7 mmol/l) or no retinopathy (4.6 +/- 0.8 mmol/l, trend test; p less than 0.003), due to raised levels of low density lipoprotein (LDL) cholesterol (3.8 +/- 0.9, 3.2 +/- 0.6 and 2.8 +/- 0.8 mmol/l respectively; p less than 0.02). High density lipoprotein (HDL) levels were similar in patients with and without retinopathy and HDL/LDL ratios were lower with more severe retinopathy (p less than 0.025). Cholesterol levels were similar in diabetic subjects without retinopathy and in 12 normal subjects. Triglyceride levels were not related to retinopathy and no measure of plasma lipids correlated with HbA1 or 24-h mean plasma glucose. Total and LDL cholesterol were weakly inversely correlated with creatinine clearance but the association with retinopathy was independent of this effect.