The aim of the study was to evaluate the efficacy of enalapril and atenolol in decreasing the severity of proteinuria in hypertensive patients suffering from insulin-dependent diabetes mellitus. We studied 20 hypertensive patients. All patients had proteinuria (> 3 g/24 h) and were receiving insulin treatment. Proteinuria was measured monthly in the run-in period (3 months) and during the active drug treatment (8 months). Glomerular filtration rate, effective renal plasma flow, filtration fraction, and total renal resistance were determined after the run-in and treatment periods. The patients were randomly assigned to treatment with enalapril 20 mg/day or atenolol 100 mg/day for 8 months. In both groups blood pressure decreased significantly. After 8 months' treatment, severity of proteinuria significantly decreased both in the enalapril-treated group and in the group receiving atenolol. Glomerular filtration rate and effective renal plasma flow significantly increased, while total renal resistance decreased in the patients given enalapril, whereas glomerular filtration rate, renal plasma flow, and total renal resistance significantly decreased in the patients given atenolol. The results of this study show that enalapril and atenolol reduce proteinuria in hypertensive diabetic patients by a mechanism related to their antihypertensive effects; furthermore, the beneficial effects of enalapril might be also linked to intrarenal effects.