In recent years several cases of anemia (iron deficiency) have been reported in adults who participate in long-distance running although its etiology has not been entirely explained. We report the case of a 16-year-old girl who had participated in middle-distance running at a competitive level for about three years and who had been admitted to hospital because of a progressive weakness and a reduction in her sporting performance. Evaluation revealed that the patient had a balanced diet, normal menstrual cycles and slight abdominal pain. The objective examination were negative except for the presence of a pronounced pallor of the skin and the mucous membranes. The blood count revealed Hb 7.5 g%, Ht 26%, GV 63 mu 3, the reticulocyte count was 10%, serum iron 9 micrograms/dl, serum transferrin 450 micrograms/dl and serum ferritin 4 ng/ml. All tests for constitutional anemia proved negative. Stool Hemoccult tests proved negative (these tests were carried out some weeks after the patient had stopped running). Her symptoms resolved after the beginning of iron treatment and her blood test results returned to normal. This case has been reported to draw attention to the existence of this problem in adolescents who practice sport. The knowledge of the problem night lead to a preventive scanning of young athletes and the presence of clinical manifestations would reduce the need for invasive tests.