A hematological comparison was performed between 43 middle and long distance male runners and 119 male controls. The hematocrit, serum iron, transferrin saturation and serum ferritin values were significantly lower in the athletes. The amount of bone marrow hemosiderin was also lower in the athletes than in a group of non-athletic men of the same age. Even if these values were clearly lower than in the controls, they were not low enough to indicate iron deficiency. The observations that sideroblast counts in bone marrow smears were normal and that both red cell indices and red cell protoporphyrin were normal strongly support the conclusion that lack of iron had not limitated erythropoiesis or the formation of an optimal red cell mass. Low serum haptoglobin values in most athletes indicated an increased intravascular hemolysis. As the hemoglobin-haptoglobin complex formed is taken up by hepatocytes, this implies that there is a shift in the red cell catabolism in these athletes from the reticuloendothelial system to the hepatocytes. This shift may explain the paradoxical findings of low serum ferritin concentrations and reduced contents of bone marrow hemosiderin. This is consistent with the observed normal erythropoiesis. It was concluded that runners "anemia" is no true anemia and not caused by iron deficiency. "Sports anemia" is thus no indication for routine iron supplementation.