It has been proposed that lowering glomerular pressure in children with insulin-dependent diabetes mellitus will reduce microalbuminuria and that this reduction may preserve renal function. We therefore conducted a double-blind, placebo-controlled, crossover trial to compare 3 months of treatment with the angiotensin converting enzyme inhibitor captopril (0.9 mg/kg/day), and 3 months of placebo administration to 12 normotensive adolescents with insulin-dependent diabetes mellitus, 11 with microalbuminuria (albumin excretion rate of 15 to 200 micrograms/min) and one with early overt nephropathy. Mean age (+/- SD) was 14.4 +/- 1.7 years, and disease duration was 5.1 +/- 2.5 years. Albumin excretion rate decreased significantly during captopril therapy (baseline 78 +/- 114 micrograms/min; mean of monthly measurements 38 +/- 55 micrograms/min vs placebo 78 +/- 140 micrograms/min; p less than 0.001). During captopril therapy, albumin excretion was reduced by 41 +/- 44% and decreased in 10 of 12 subjects, but was unchanged in two, one with a borderline albumin excretion rate (16.3 micrograms/min) and one with diabetes of short duration (2.9 years). Plasma renin activity rose significantly during captopril therapy, and mean arterial pressure decreased slightly (placebo 81 +/- 7 mm Hg; captopril 76 +/- 5 mm Hg; p = 0.004). After 3 months of captopril treatment, glomerular filtration rate and renal plasma flow did not change significantly. Hemoglobin Alc values remained stable during the study. The only side effect of captopril was diarrhea in one patient. We conclude that, in the short term, captopril is effective in decreasing albumin excretion rate in normotensive children with insulin-dependent diabetes mellitus and microalbuminuria, without significant side effects. Longer trials are indicated in an attempt to delay or prevent overt nephropathy.