We aimed to examine the relationship of serum lipids, lipoproteins, apolipoproteins and antioxidants with renal dysfunction as measured by urinary excretion of albumin and of retinol binding protein (REP) in insulin-dependent diabetes mellitus (IDDM). We studied 121 patients with IDDM. Glomerular function was assessed as the urinary albumin/creatinine ratio (U-A/U-C), and tubular function as the urinary retinol-binding protein/creatinine ratio (U-R/U-C), both measured in three early morning spot urine samples. The mean (range) U-A/U-C was 1.95 mg/mmol (0.3-476.5) and U-R/U-C was 17.5 mu g/mmol (1.0-1853.8). 17% of the patients had a U-A/U-C > 3 mg/mmol and 33% had a U-R/U-C > 20 mu g/mmol. Significant positive 'correlations were observed between both U-A/U-C and U-R/U-C and the following: serum total cholesterol (P <0.005); triglycerides(P <0.001); apolipoproteins A-I (P <0.05), A-II (P <0.02) and B (P <0.002); glycated haemoglobin (P <0.002). No significant associations were found with serum vitamin E, beta-carotene or total antioxidant activity. In multiple regression, only U-A/U-C was independently associated with serum apo B and cholesterol concentrations. In conclusion, in IDDM glomerular dysfunction, as measured by U-A/U-C, is associated with elevated serum cholesterol, triglycerides, apo B, apo A-I and apo A-II, but not with HDL cholesterol or antioxidant status. Tubular dysfunction tends to occur with increasing albuminuria, but it is not independently associated with serum lipid, lipoprotein, apolipoprotein or antioxidant levels.