OBJECTIVE: To assess if low (1.25 mg) and/or standard (5 mg) doses of the ACE inhibitor ramipril could prevent progression of microalbuminuria (incipient diabetic nephropathy) in normotensive type 1 diabetic patients. RESEARCH DESIGN AND METHODS: This study, using a multicenter randomized placebo-controlled double-blind parallel group, was conducted over 2 years in 28 outpatient diabetic clinics in the U.K. and Ireland. We screened 334 type 1 diabetic patients with suspected microalbuminuria and normal blood pressure; of these, 140 patients 18-65 years of age with a diagnosis of type 1 diabetes and persistent microalbuminuria, defined as urinary albumin excretion rate (AER) of 20-200 microg/min, were enrolled in the study. RESULTS: The proportion of patients progressing to macroalbuminuria was reduced in the ramipril groups but did not reach statistical significance over 2 years. AER was significantly lower at year 2 in the combined ramipril-treated patients versus placebo (P = 0.013). More patients on ramipril regressed to normoalbuminuria (<20 microg/min), with 11% for 1.25 mg ramipril, 20% for 5 mg ramipril, and 4% for placebo (P = 0.053). Blood pressure was significantly reduced to a similar extent with both 1.25 and 5 mg ramipril. Supine systolic blood pressure increased from 130 to 134 mmHg in the placebo group and fell in the 1.25 mg ramipril group (from 132 to 129 mmHg) and in the 5 mg ramipril group (from 134 to 130 mmHg) (P = 0.003, compared with placebo). No statistically significant changes were observed in glomerular filtration rate (GFR) between the placebo- and ramipril-treated groups during the 2-year period. CONCLUSIONS: Microalbuminuria is reduced significantly by ramipril treatment in type 1 diabetic patients without hypertension. Although the magnitude of the response was greater, there is no significant difference between responses to 1.25 or 5 mg ramipril. Small but highly significant reductions in systolic and mean arterial pressures occur in ramipril-treated patients. GFR is stable at this stage of the evolution of diabetic nephropathy and is unaffected by ramipril treatment for 2 years.