The relationships between serum lipid, apolipoprotein levels and urinary albumin excretion were investigated in 20 male Type 1 (insulin-dependent) diabetic patients with microalbuminuria (overnight urinary albumin excretion between 10 and 200 micrograms/min), in 18 male Type 1 diabetic patients without microalbuminuria and in 18 male control subjects. In the microalbuminuric patients low density lipoprotein cholesterol was higher than in the control subjects (p less than 0.05); the high density lipoprotein/low density lipoprotein cholesterol ratio was lower than in the normalbuminuric diabetic patients (p less than 0.05), and in the control subjects (p less than 0.01); apolipo-protein B was higher than in the normalbuminuric patients (p less than 0.05); the apolipoprotein A1/B ratio was lower than in the normoalbuminuric diabetic patients (p less than 0.05). Serum triglyceride was higher in the microalbuminuric diabetic patients and in the control subjects than in the normalbuminuric diabetic patients (p less than 0.05, for both), but was not different between the microalbuminuric diabetic patients and the control subjects. No significant differnces between the 3 groups were present with respect to serum cholesterol, high density lipo-protein cholesterol and apolipoprotein A1. In the 2 combined Type 1 diabetic groups there were significant correlations between urinary albumin excretion and the high density lipoprotein/low density lipoprotein cholesterol ratio (R -0.40, p less than 0.02), apolipoprotein B (R 0.35, p less than 0.05) and the apolipoprotein A1/B ratio (R -0.44, p les than 0.01). These results indicate microalbuminuria related differences in lipid and apolipoprotein levels in male Type 1 diabetic patients, which may contribute to an increased risk of cardiovascular disease.