Diabetic nephropathy is the most frequent cause of chronic renal failure. The onset of microalbuminuria in patients with diabetes mellitus, which seems to be related to blood pressure and the control of glycemia, is predictive of the development of true proteinuria. This multicenter, single-blind, randomized study examined the effects of benazepril and nicardipine on overnight microalbuminuria in 57 normotensive and 46 hypertensive diabetic patients. At the end of a 3-month placebo run-in period, the patients were stratified on the basis of the presence or absence of arterial hypertension and, within each stratum, randomized to receive one daily tablet of 10 mg benazepril or one tablet of 20 mg nicardipine twice daily for 6 months. Renal hemodynamics was investigated in 25 patients. Both drugs decreased overnight microalbuminuria throughout the study period, but benazepril was more effective than nicardipine (p = 0.025); in the patients with hypertension, both drugs led to a similar marked reduction in systolic and diastolic blood pressure. This study shows that benazepril was more effective than nicardipine in reducing overnight microalbuminuria in patients with diabetes mellitus, independently of their antihypertensive properties.