Iron status was studied in 126 female endurance athletes and 52 control subjects, all aged 16-20 years. The study aimed at identifying factors responsible for iron deficiency. Twenty-six percent of athletes and 50% of controls had latent iron-deficiency without anemia symptoms. A too low intake of iron (especially heme iron: 0.3 mg daily), and of nutrients influencing iron metabolism, were identified as main causes of iron deficiency in control subjects. In athletes, whose iron intake was sufficient (14.6 mg), the principal cause of iron deficiency were blood losses due to menstruation. High level of physical activity, expressed as training volume and experience, did not adversely affect iron stores, as these were higher than in control subjects and the incidence of iron deficiency was much lower than in the control group. It was concluded that an increased intake of iron and of dietary factors involved in iron metabolism prevented possible exercise-induced losses of iron in young athletes.