Seventy-two high school cross-country runners were studied during the running session for possible etiologic factors associated with iron deficiency, which was defined as a serum ferritin level less than or equal to 12 ng/ml and a transferrin saturation of less than or equal to 16% occurring simultaneously. Iron deficiency was observed during the running season in 34% of female cross-country runners, compared with 8% of male runners. Increased iron losses through gastrointestinal bleeding occurred in 9 of 20 female runners; 7 of these 9 had iron deficiency. Dietary iron intake was low in both iron-deficient and iron-sufficient female runners, but dietary instruction did not increase iron intake significantly. Iron deficiency could not be prevented in 35% of the female runners treated with 60 mg of elemental iron daily, but adequate treatment was achieved with 180 mg. Iron losses in urine, sweat, and plasma were small and did not appear to be increased in iron-deficient runners. These findings indicate that female cross-country runners have a high incidence of iron deficiency that is associated with initially decreased iron stores and gastrointestinal bleeding.