The beneficial effect of long-term treatment with an angiotensin-converting enzyme (ACE) inhibitor on urinary microalbumin excretion (UAE) and renal function was investigated in a 4 year, randomized prospective study in normotensive patients with non-insulin-dependent (Type 2) diabetes mellitus. Sixty-two normotensive patients with Type 2 diabetes mellitus and microalbuminuria but normal renal function were randomized to receive either enalapril 5 mg day-1 or no treatment. In the enalapril-treated patients, UAE was reduced from 115.4 +/- 80.1 to 95.6 +/- 61.7 mg 24 h-1 after 12 months (p < 0.05) and to 75.3 +/- 44.8 mg 24 h-1 after 48 months (p < 0.001). In the untreated group, UAE increased slowly from 93.9 +/- 69.9 to 150.0 +/- 144.5 mg 24 h-1 after 48 months. No changes in creatinine clearance, blood pressure or HbA1C were seen in either group during the 4-year period. In normotensive Type 2 diabetic patients with early stage of diabetic microalbuminuria. This effect is long-lasting and probably independent of the antihypertensive action of the drug.