Postexercise proteinuria has been observed in healthy adults but there was a need for information in young individuals. Boys and girls (n = 170) from 6 to 18 years of age were submitted to maximal exercise by the 20-meter shuttle run test. Urine collection was made prior to and 30 min after completion of the run. Total protein, albumin, beta 2-microglobulin (beta 2-m), retinol-binding protein (RBP) and creatinine were determined on each sample. Resting values for total protein, albumin, beta 2-m and RBP excretion rates were within the normal range for each component with a progressive increase for total protein and albumin. The effect of exercise increased with age. Between the 6 and 9 year olds there was a gender difference in postexercise proteinuria, boys enhanced the excretion of macromolecules while girls did not show any increase by maximal exercise. The excretion rates of all protein components were related to the absolute intensity of exercise expressed as maximal speed (R between 0.86 and 0.90, p < 0.001). High- and low-molecular weight protein excretion gave evidence of increasing disturbances from the age of 9 to 18 years in boys and girls. These data clearly demonstrate that 1) postexercise proteinuria is present at maximal exercise from childhood to adolescence; 2) the magnitude of protein excretion is strictly related to the absolute intensity of exercise; 3) the glomerulus permeability and the tubular reabsorption process are both different in pre- and postpubertal children and adolescents.