To determine whether inhibition of angiotensin converting enzyme can reduce the rate of decline in kidney function more than reducing blood pressure with other antihypertensive treatment. Prospective, open randomised study lasting a mean of 2.2 years in patients with diabetic nephropathy. Three outpatient nephrology clinics. 40 patients with insulin dependent diabetes and diabetic nephropathy with reduced renal function. Antihypertensive treatment with enalapril or metoprolol, usually combined with frusemide. Rate of decline in glomerular filtration rate measured as chromium-51 edetic acid clearance. Glomerular filtration rate declined a mean of 2.0 (SD 3.2) ml/min/year in the group given enalapril and 5.6 (5.9) ml/min/year in the control group. The mean arterial blood pressure during the study was 102 (5) mm Hg in the patients given enalapril and 103 (5) mm Hg in the patients given metoprolol. Urinary albumin excretion during treatment with enalapril was 60% lower than during treatment with metoprolol. Enalapril has an antiproteinuric effect independent of the effect on systemic blood pressure. Treatment with enalapril can reduce the rate of decline in kidney function in patients with diabetic nephropathy more than equally effective antihypertensive treatment with metoprolol. This points to a specific renal protective effect of angiotensin converting enzyme inhibitors in diabetic nephropathy.