We compared the efficacy of treatment protocols with an angiotensin converting enzyme (ACE) inhibitor alone (enalapril, 5 mg) or angiotensin II (ATII) receptor blocker (losartan, 50 mg) or both enalapril plus losartan in patients with microalbuminuria in a prospective, randomized clinical trial. Normotensive type 2 diabetic patients with microalbuminuria documented by at least 3 consecutive urinary albumin excretion analyses were recruited for the study. Patients were grouped randomly into one of the protocols which consisted of treatment with 5 mg enalapril daily (group 1; n=12), 50 mg losartan daily (group 2; n=12) or both drugs (group 3; n=10). They were reevaluated with regard to HbA1c levels, lipid profiles, blood pressure and urinary albumin excretion rates (UAER) at 3-month intervals for 12 months. Mean age, duration of diabetes, body mass index, plasma lipid profiles and blood pressure levels were similar at the initial visit. In group 1, UAER returned to normal levels in 10 patients. Normalization of UAER occurred in 8 and 7 patients in groups 2 and 3, respectively. Percentage of reduction in UAERs at the end of 12 months were 58%, 59% and 60% (p=0.0001; p=0.0002; p=0.0003, respectively). The amount of reduction in UAER did not differ significantly among the three groups (p=0.346). ACE inhibitors and angiotensin II receptor blockers have similar efficacy in treating diabetic microalbuminuria, and the combination of the two drugs does not add any further benefit.