Background/Aims: Exercise-induced proteinuria is a common consequence of physical activity, although its mechanism is not clear. Oxidant stress has been proposed as one of different factors involved in postexercise proteinuria in rats. In this study we investigated whether reactive oxygen radicals generated during exercise play a role in exercise-induced proteinuria in sedentary and trained men. Methods: The validity of oxidant stress following stepwise maximal exercise on proteinuria was investigated in sedentary and trained subjects before and after antioxidant vitamin treatment (A, C, and E) for 2 months. While protein carbonyl content in serum and thiobarbituric acid reactive substances (TBARS) in erythrocytes and urine were used as oxidant stress markers, total protein, albumin, β<sub>2</sub>-microglobulin in urine were assayed for proteinuria in five consecutive specimens after exercise. Urines were collected before exercise, then 30 min, 2, 8 and 24 h postexercise. Results: Increased urinary protein levels and mixed type proteinuria were determined after 30 min of exercise in sedentary and trained subjects. Proteinuria was normalized at 2 and 8 h specimens. However, glomerular type proteinuria was identified at 24 h specimen in both groups. Oxidant stress markers were significantly elevated in sedentary and trained subjects. Antioxidant treatment prevented the increase in oxidant stress markers, urinary protein levels and the occurrence of glomerular type proteinuria after exhaustive exercise at 24 h in both groups. Conclusions: These findings suggest that the exercise-induced oxidant stress may contribute to exercise-induced proteinuria in sedentary and trained men.