In the present study the effect of angiotensin converting enzyme inhibitor captopril was studied in normotensive diabetic children and adolescents with persistent microalbuminuria (repeated albumin excretion rate higher than 30 mg/24 h). In 1993/1994, 15 microalbuminuric patients (age: 10-17 yrs, diabetes duration: 6.5 +/- 3.0 yrs) had been treated with captopril (0.9 mg/kg/day for a period of 13.1 +/- 4.4 months). In 1992/1993, 13 patients (age: 11-17 yrs, diabetes duration: 5.8 +/- 2.7 yrs, study period: 12.3 +/- 4.0 months) had not received captopril. Same restriction of the dietary protein intake was recommended in both groups (less than 10% of the total calorie intake). Timed 24-h urine samples were used to determine albumin excretion by an immunonephelometric method. Significant increase in microalbuminuria was observed in patients who had not received captopril during the study period (56.2 +/- 16.0 mg/24 h vs. 77.8 +/- 20.1 mg/24 h, p = 0.045). There was no change in microalbuminuria in the captopril treated group during the study period (60.6 +/- 19.5 mg/24 h vs. 60.4 +/- 25 mg/24 h, n. s.). No change in metabolic control and blood pressure was observed in the two groups during the study period. These results support that captopril treatment may prevent or delay the progression of incipient nephropathy in normotensive children and adolescents in diabetes.