In brief: As many as 80% of female long-distance runners show iron deficiency. One explanation is RBC fragmentation due to mechanical trauma. In this study, radiolabeling was used to measure RBC survival rates in six iron-replete female marathon runners. In addition, tests for urinary hemosiderin were conducted in a search for secondary evidence of RBC damage. Radioactivity counts on blood samples drawn before and after a standard 26-mile marathon failed to disclose traumatic RBC fragmentation, and the tests for urinary hemosiderin were negative. Other possible factors in iron deficiency, such as diet, therefore merit consideration.