We questioned the superiority of angiotensin converting enzyme (ACE) inhibitors to beta blocking drugs with regard to renal function outcome in patients with mild to moderate renal insufficiency and normal to moderately elevated blood pressure (BP). We therefore studied 89 patients in a prospective double-blind randomized trial comparing the effect of enalapril and atenolol on the slope of glomerular filtration rate (GFR). Mean baseline GFR was 53 +/- 20 ml/min, untreated BP 152 +/- 20 mm Hg systolic and 90 +/- 11 mm Hg diastolic and median proteinuria 0.6 g/24 hr (interquartile range 0.0 to 2.5). After a run-in period without antihypertensives, the test drug was titrated to lower diastolic BP to a predefined goal of 10 mm Hg below baseline and/or below 95 mm Hg. The median follow up was 3.9 years. Antihypertensive therapy resulted in a comparable decrease of BP in both study groups. Filtration fraction and proteinuria decreased in both groups. The slope of GFR over time was not different between both groups (-1.39 +/- 2.82 and -1.97 +/- 3.38 ml/min/year on atenolol and enalapril, respectively). In multiple regression analysis a higher baseline GFR, a greater decrease in GFR and in proteinuria during titration and a lower proteinuria during follow up were independently related to a better GFR slope. We conclude that the use of ACE inhibitors is not imperative in all patients with non-diabetic nephropathy.