OBJECTIVE To assess the prevalence of microalbuminuria (> or = 30 mg/24 h) in children with insulin dependent diabetes mellitus (IDDM). METHODS Retrospective. METHODS Department of Pediatrics, University Hospital, Free University of Amsterdam. METHODS Albumin excretion rate was assessed in 24-hour voiding volumes in 71 children, mean age 11.8 yrs (range 3.5-17.9; 27 boys and 43 girls). Mean duration of IDDM was 5.4 yrs (1.0-5.0). Patients with albuminuria were compared with patients without microalbuminuria. Results of angiotensin-converting enzyme inhibition (n = 4) are presented. RESULTS The prevalence of microalbuminuria was 16% (n = 11). In 7% (n = 5) the microalbuminuria was persistent; in 9% (n = 6) it was intermittent. The mean age of the children with microalbuminuria (14.0 years; 8.9-17.4) was significantly higher than the age of the children without this disorder (11.7 years; 3.5-17.9). Two prepubertal children had developed microalbuminuria. The group with microalbuminuria had a significantly higher age at onset of the disease (8.1 years; 1.9-13.0) than the group without microalbuminuria (5.8 jaar; 0.9-14.5). The daily dose of insulin in the group with microalbuminuria was significantly higher (1.15 U/kg; 0.6-1.56) than in the group without microalbuminuria (0.97 U/kg; 0.20-1.87). There was no correlation between microalbuminuria and the duration of the disease. CONCLUSIONS Microalbuminuria may exist as early as the prepuberal period, but is found more frequently in children with IDDM who have entered puberty. There is a correlation between microalbuminuria and the age which it develops, the age at onset of the IDDM and the daily dose of insulin. Secondary prevention of diabetic nephropathy with ACE inhibition appears to be possible.