The aim of the present study was to investigate the renal effects of long-term treatment with the calcium channel blocker nifedipine in normotensive type 1 diabetic patients with microalbuminuria. In a randomized, double-blind trial, 15 type 1 diabetic patients were treated with either nifedipine (n = 8; dosage 30 mg/day) or placebo (n = 7) for 12 months. At baseline and after 6 and 12 months of therapy, the albumin excretion rate (UAER, radioimmunoassay), glomerular filtration rate (GFR, chromium 51 ethylenediamine tetra-acetic acid clearance) and renal plasma flow (RPF, iodine 125 hippuran clearance) were determined. Nifedipine treatment caused a significant reduction of UAER after 6 and 12 months (median, Q1/Q3 in mg/24 h): baseline 84 (65/163); 6 months 35 (23/90), P < 0.02; 12 months 39 (15/79), P < 0.05). GFR was significantly decreased by nifedipine treatment (baseline 157 +/- 15, 6 months 122 +/- 8, 12 months 111 +/- 47 ml/min; P < 0.05, mean +/- SEM), whereas RPF remained constant. Nifedipine treatment did not influence systolic (baseline 121 +/- 7, 12 months 124 +/- 2 mmHg, mean +/- SEM) or diastolic (baseline 72 +/- 2, 12 months 74 +/- 3 mmHg) arterial blood pressure. With placebo treatment no significant alterations of UAER, GFR, RPF and arterial blood pressure were observed. Metabolic control was constant throughout the whole study period. Thus, 1 year's treatment with nifedipine reduces the UAER and GFR in normotensive type 1 diabetic patients without influencing the systemic arterial blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)