About 30% of diabetic patients develop progressive renal failure. We studied albumin, IgG, and transferrin excretion during exercise in diabetic children without signs of nephropathy to investigate proteinuria under these conditions: 39 patients with insulin-dependent diabetes mellitus and 21 healthy children undertook a bicycle exercise test. Albuminuria measured by nephelometry was calculated as the albumin excretion rate (AER) and albumin-to-creatinine ratio before and after exercise. The diabetic group was divided into three subgroups according to disease duration (DI<5 years, DII 5–10 years, DIII>10 years). No significant difference in metabolic control (hemoglobin A_1c) was detected between the diabetic groups (median hemoglobin A_1c: DI 7.2%, DII 7.6%, DIII 8.6%). There was no increase in AER in the healthy children after exercise. Before exercise the diabetic groups had an AER similar to controls. No significant increase in albuminuria after exercise was seen in group DI. Both groups with a disease duration of more than 5 years had a significant increase in albuminuria [median before/after: DII 7.8/16.7 (P<0.05), DIII 0/57.9 (P<0.05) ώg/min per 1.73 m²). Of these patients, 43% also had a measurable urinary excretion of IgG and transferrin, indicating structural glomerular damage. There was no correlation of albuminuria and parameters of metabolic control or renal function. We conclude that in diabetic children an exercise test unveils albuminuria in certain patients, while their AER may be normal at rest.