To verify the hypothesis that angiotensin-converting enzyme (ACE) inhibitors possess a unique renoprotective effect in progressive chronic renal disease, we decided to compare the effects of an ACE inhibitor and a calcium antagonist on both hypertension and the progression of non-diabetic renal insufficiency in a long-term study. A four-year, multicenter, prospective, randomized trial was conducted on 142 hypertensive patients (pts) with established chronic renal failure from six Italian nephrology departments. They were on standard antihypertensive therapy with a low-protein diet and underwent twice-monthly surveillance for a one year pre-randomization period. After that year, 121 pts were randomly allocated to captopril or slow-release nifedipine therapies for a three-year study period. The progression of renal insufficiency was monitored every two months. Blood pressure control was significantly better after randomization than during the year of standard antihypertensive therapy. The progression rate before randomization (BR) was definitely higher before than after randomization (AR): Creatinine clearance (CCr) change BR = -0.46 +/- 0.45 ml/min/month, creatinine clearance change AR = -0.23 +/- 0.43 ml/min/month (P less than 0.01). After randomization, the mean blood pressure values were virtually the same throughout the three year period of the study in the two groups treated by captopril (group I), or nifedipine (group II).(ABSTRACT TRUNCATED AT 250 WORDS)