To examine the effect of low-dose losartan, an angiotensin II antagonist, on persistent microalbuminuria in normotensive Type 1 diabetes mellitus, 16 subjects with Type 1 diabetes were randomly assigned to two 2-month treatment periods, with either losartan (25 mg/day) or enalapril (5 mg/day) in a single-blind cross-over design. Urinary albumin excretion (UAE), blood pressures, lipids, glycemia, HbA1C, serum potassium and creatinine clearance were measured before and after each treatment period. The UAEs were similarly reduced after both treatments. The median UAE decreased by 27.8%, from 162 (range 65-250) to 117 (34-190) mg/day (p<0.01) after enalapril, and decreased by 25%, from 160 (60-246) to 120 (36-184) mg/day (p<0.01) after losartan. The systolic and diastolic blood pressures also decreased significantly (p<0.05), whereas serum levels of potassium increased (p<0.01) after both treatments. The levels of serum HbA1c, mean fasting glucose, total cholesterol, triglycerides, LDL cholesterol, HDL cholesterol and creatinine clearances were not significantly (p>0.05 in all) changed by either the enalapril or losartan treatment. No significant differences were found between the effects of enalapril and losartan. In conclusion, losartan treatment reduces microalbuminuria as effectively as enalapril in normotensive Type 1 diabetic patients.