Nation-wide screening for microalbuminuria in Denmark was performed in 22 paediatric departments treating children with Type 1 diabetes. Over a period of 6 months 1020 children (less than or equal to 12 years) and adolescents (greater than 12 to 19 years) were screened (81% of total). Of these, 957 (94%) performed at least two timed overnight urine collections. In 209 non-diabetic subjects the upper 95% limit for normal albumin excretion rate (AER) was 20 micrograms min-1. Mean overnight AER was significantly (p less than 0.001) elevated in diabetic (3.0 x/divided by 2.3 (SD tolerance factor) micrograms min-1) and in non-diabetic (2.5 x/divided by 2.2 micrograms min-1) adolescents compared with diabetic (1.7 x/divided by 2.1 micrograms min-1) and non-diabetic (1.3 x/divided by 2.2 micrograms min-1) children. In the diabetic patients AER was positively correlated with the body surface area and age. Among the patients with Type 1 diabetes, 4.3% (18 males and 23 females) had AER greater than 20 to 150 micrograms min-1 (persistent microalbuminuria). A further 7 adolescents (0.7%) had overt proteinuria (greater than 150 micrograms min-1). Clinical data for the 41 diabetic patients with AER greater than 20 to 150 micrograms min-1 were compared with those for 569 diabetic adolescents with AER less than or equal to 20 micrograms min-1 and duration of diabetes more than 2 years. The group with AER greater than 20 to 150 micrograms min-1 had significantly higher mean age (16.5 years) than the group with AER less than or equal to 20 micrograms min-1 (15.0 years; p less than 0.001). Females with AER greater than 20 to 150 micrograms min-1 had significantly higher mean HbA1c level (10.8 +/- 1.9%) than those with AER less than or equal to 20 micrograms min-1 (9.8 +/- 1.9%, p less than 0.003); they also had impaired linear growth (standard deviation score -0.25 vs + 0.16; p = 0.003). These associations were not found in males. Mean body mass index (BMI) was significantly increased in both females (22.2 +/- 2.9 kg m-2) and males (20.8 +/- 2.7 kg m-2) with AER greater than 20 to 150 micrograms min-1, compared with diabetic patients with AER less than or equal to 20 micrograms min-1 (females 20.8 +/- 3.0 kg m-2, p = 0.02; males 19.7 +/- 2.4 kg m-2, p less than 0.006).(ABSTRACT TRUNCATED AT 400 WORDS)