Serum ferritin, transferrin, haptoglobin, and iron were measured in well-trained middle- and long-distance runners, elite rowers of the West German national team, and professional racing cyclists during the summer training and the winter rest period. None of the male athletes examined, with the exception of the racing cyclists during the summer period, received oral or parenteral iron. The runners were found to have significantly lower ferritin (P less than 0.00001), iron (P less than 0.001), and haptoglobin values (P less than 0.01) than the controls. Their transferrin levels were elevated, however not significantly. Rowers showed significantly higher ferritin levels (P less than 0.01) than the controls. The reduced haptoglobin concentrations in runners are presumed to be caused by a running-induced hemolysis. It is speculated that a recurring hemoglobinuria produced diminished iron reserves in middle- and long-distance runners.