The albumin excretion rate (AER) was studied in two groups of diabetic children and adolescents. Twenty-four-hour AER was studied in 75 children with diabetes for 5 years, in 49 children with diabetes for 10 years, in 55 children with diabetes for 10-20 years and in 21 age matched healthy controls. Overnight AER was studied in 129 diabetic children and adolescents with a duration of diabetes varying from 1-14 years. Diabetics exhibited a wide range of AER-values and when expressed per body surface area, diabetic children had significantly higher AER compared to controls. Log transformed AER-values were significantly correlated to age and body surface area in diabetics but not in controls. In the diabetics, log AER was also correlated to systolic and diastolic blood pressure but not to HbA1c. 20% of the diabetics had AER values exceeding the upper value for healthy controls which was 18.5 micrograms/min. 31/35 of them were older than 12 years. In both groups of diabetics, 5% had AER-values exceeding those reported to be predictive for later development of overt nephropathy, the youngest being 16 years old. When comparing diabetic children 0-12 years (i.e. before the maximal growth spurt of puberty) to those older than 12 years, at the same duration of diabetes, the latter group had significantly higher AER-values. No sex difference was found in either age group. It is concluded that after puberty diabetic patients also show evidence of incipient diabetic nephropathy. Thus, routine screening for microalbuminuria is recommended also in pediatric diabetes care after 12 years of age.