The influence of simvastatin, a competitive inhibitor of 3-hydroxy-3-methyl glutaryl coenzyme A reductase, on quantitative and qualitative changes in lipoprotein metabolism was investigated in 18 patients (group I, 10 with primary kidney disease and group II, 8 with diabetic nephropathy) with nephrotic syndrome. Nephrotic patients exhibited severe hyperlipidemia (serum cholesterol 390 +/- 17 mg/dl and triglyceride 335 +/- 42 mg/dl; mean +/- SEM) and had significantly higher lipoprotein (a) [Lp(a)] levels (54 +/- 12 mg/dl; median 31 mg/dl, p < 0.01) compared with 20 healthy subjects (mean 12 +/- 1.8 mg/dl; median 7 mg/dl). Fifty-six percent of the patients and 15% of the controls had values greater than 30 mg/dl. Treatment with simvastatin in increasing doses over a period of three months (13 patients received 40 mg/day and 5 patients 20 mg/day at the end of the third month) reduced LDL-cholesterol in both groups of patients (35% and 54%) as well as apolipoprotein B (apoB) (31% and 46%) significantly, but Lp(a) levels were not influenced (57 +/- 21 vs 59 +/- 20 and 50 +/- 14 vs 53 +/- 16 mg/dl, respectively). On the other hand a complex change in lipoprotein composition occurred. The ratio of LDL apoB/LDL cholesterol-ester increased significantly (0.75 +/- 0.03 to 0.84 +/- 0.03 and 0.80 +/- 0.03 to 1.02 +/- 0.1, respectively) and cholesterol concentration in VLDL (64 +/- 16 to 39 +/- 7 and 74 +/- 18 to 55 +/- 74 mg/dl, respectively) was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)