The aim of this study was to assess the effect of simvastatin on plasma lipoproteins and renal function in hypercholesterolaemic Type 1 (insulin-dependent) diabetic patients with diabetic nephropathy. Twenty-six hypercholesterolaemic (total cholesterol greater than or equal to 5.5 mmol/l) Type 1 diabetic patients with nephropathy were enrolled in a double-blind randomized placebo-controlled study for 12 weeks. The active treatment group (n = 14) received simvastatin (10-20 mg/day) for 12 weeks while the remaining 12 patients received treatment with placebo. The results during simvastatin treatment (baseline vs 12 weeks): total cholesterol 6.6 vs 4.8 mmol/l (p less than 0.01), LDL-cholesterol 4.25 vs 2.57 mmol/l (p less than 0.01) and apolipoprotein B 1.37 vs 1.06 mmol/l (p less than 0.01). HDL-cholesterol, and apolipoprotein A-I remained unchanged. Total cholesterol, LDL-cholesterol, HDL-cholesterol, apolipoprotein A-I, apolipoprotein B remained unchanged during placebo treatment. Albuminuria measured during the simvastatin and the placebo treatment (baseline vs 12 weeks) (the data are logarithmically transformed before analysis because of their positively skewed transformation; geometric mean (x/divided by antilog SE) is indicated) was 458 (x/divided by 1.58) vs 393 (x/divided by 1.61) and 481 (x/divided by 1.62) vs 368 (x/divided by 1.78 micrograms/min (NS). Glomerular filtration rate during simvastatin and placebo treatment (baseline vs 12 weeks) was 64 vs 63 and 72 vs 74 ml.min-1.1.73 m-2, respectively. Two patients receiving simvastatin treatment were withdrawn, one due to gastrointestinal side effects and one due to myalgia.(ABSTRACT TRUNCATED AT 250 WORDS)