OBJECTIVE To determine whether long-term treatment with an angiotensinconverting enzyme (ACE) inhibitor has a beneficial effect on the urinary microalbumin excretion and renal function in non-insulin-dependent diabetes mellitus (NIDDM) patients, enalapril (5 mg/day) was administered for 48 months. RESEARCH DESIGN AND METHODS Fifty-two patients with NIDDM who had persistent microalbuminuria in the range of 20–300 mg/24 h, serum creatinine < 106.1 μM 1.2 mg/dl), supine systolic blood pressure (BP) 1c P < 0.005), whereas no significant change occurred in group NC. In the well-controlled hypertensive groups, a significant reduction in UAE occurred in group HE (P < 0.05), whereas no significant change occurred in group HC. No changes in creatinine clearance, BP, or blood glucose control were seen during the study. CONCLUSIONS Treatment with enalapril for 48 months may have a beneficial effect on the decline of microalbumin excretion in NIDDM patients.