The relationship between serum lipid, lipoprotein, and apolipoprotein levels and abnormalities of renal function has been investigated in 112 insulin-dependent (type I) diabetic patients. They were subdivided into three matched groups according to the amount of albuminuria: group A (albuminuria less than 20 micrograms/min), group B (albuminuria between 20 and 150 micrograms/min; Albustix negative), and group C (albuminuria greater than 150 micrograms/min; Albustix positive). Twenty-one nondiabetic subjects with albuminuria above 150 micrograms/min but without nephrotic syndrome and/or renal failure and 77 healthy subjects were also studied. Mean total and LDL cholesterol, triglycerides, and apo B were higher, while HDL cholesterol and HDL/LDL cholesterol ratio were lower in group C than in groups A and B; the apo A/apo B ratio was lower in group C than in group A. Differences in apo B and in apo A/apo B ratio were found between groups A and B. No correlation between lipid parameters and amount of albuminuria was observed. Significant differences in lipid concentrations were also found in diabetic patients when compared with nondiabetic subjects with albuminuria and with healthy subjects. The present study confirmed previous reports of lipid disorders in insulin-dependent (type I) diabetes; however, the most important observation was the finding of albuminuria-related differences in lipid parameters in diabetic patients without renal failure. We think that the greater lipid abnormalities observed in diabetic patients with larger amounts of albuminuria might be the consequence both of impairment of glomerular permeability and of the diabetic state.