Twenty-four hour urinary albumin excretion was measured in 97 children with Type 1 (insulin-dependent) diabetes and found to have a geometric mean of 6 mg/day (range 1-38 mg/d). The same geometric mean of 6 mg/day (range 1-45 mg/d) was found in 120 normal children. The relationship of 24-h urinary albumin excretion to the albumin concentration (mg/l) and to the ratio of albumin: creatinine (mg:mmol) on first morning urine samples in 64 patients was highly significant (r = 0.93 and r = 0.62 respectively, p less than 0.001). In 41 patients, the relationship between 24-h urinary albumin excretion and albumin concentration upon urine samples at various times was assessed. The correlation was highest on the first morning sample (r = 0.86); 09.00 h to 13.00 h, 0.51; 13.00 h to 18.00 h, 0.68; 18.00 h to 23.00 h, 0.32. High sensitivity and moderate specificity was obtained using a first morning albumin concentration of greater than 20 mg/l to detect increased albumin excretion. These results show that the measurement of albumin concentration on the first morning urine sample can be used for a screening test for microalbuminuria in children.