The prevalence of microalbuminuria was determined in children aged 7 to 18 years with Type 1 (insulin-dependent) diabetes of more than 2 years' duration. All patients (n = 102) attending 2 diabetes clinics were asked to collect 2 overnight timed urine samples for albumin analysis by radioimmunoassay. Complete urine collection was obtained in 97 patients (95%). Overnight urinary albumin excretion rates were also measured in 36 healthy children matched for age and sex. Nineteen of the 97 patients (20%) had microalbuminuria, i.e. overnight urinary albumin excretion rates above the upper normal level (14 micrograms/min) in both urine collections. Microalbuminuria was only demonstrated in patients aged greater than or equal to 15 years, prevalence 37% (19/52 patients). Arterial blood pressure was elevated, mean 122/84 +/- 11/9mmHg, in the microalbuminuric group (19 patients) compared to the age-matched normoalbuminuric diabetic group (33 patients), mean 117/74 +/- 10/10mm Hg, p less than 0.001. The prevalence of simplex retinopathy was identical in these two groups, i.e. 25%. Glycosylated haemoglobin was slightly higher in the microalbuminuric patients, p less than 0.10. Our cross-sectional study reveals a high prevalence (37%) of persistent microalbuminuria, a stage highly predictive of later development of diabetic nephropathy, in Type 1 diabetic children aged greater than or equal to 15 years.