Low serum ferritin concentrations are commonly found in female athletes. By studying the effects of an 8-week iron or placebo supplementation in 31 female athletes (aged 17-31 years), with an initial serum ferritin concentration less than or equal to 25 micrograms/l and blood hemoglobin 120 g/l, we investigated whether low serum ferritin values hinder aerobic performance. Serum ferritin concentration increased from 14 (25th and 75th percentile: 11, 21) to 26 (18, 36) micrograms/l in the iron-supplemented group, but remained at a low 11 (9, 17) micrograms/l in the placebo group (group difference after supplementation: p = 0.001). Before supplementation, blood hemoglobin concentration was not different in the two groups. After supplementation, however, the concentration in the iron group was 139 (135, 144) g/l and 128 (126, 134) g/l in the placebo group (group difference: p = 0.001). Iron supplementation did not affect blood lactate concentration or VO2max during an incremental ergometer test. Hence, aerobic performance was not impaired in nonanemic female athletes with serum ferritin 25 micrograms/l.