The purpose of this study was to find out whether iron repletion leads to an increase in red blood cell volume (RBV) and performance capacity in iron-deficient nonanemic athletes. 40 young elite athletes (13-25 yr) with low serum ferritin (< 20 microg.L-1) and normal hemoglobin (males > 13.5 g.dL-1, females > 11.7 g.dL-1) were randomly assigned to 12-wk treatment with either twice a day ferrous iron (equivalent to 2 x 100 mg elemental iron) or with placebo using a double blind method. Before and after treatment, hematological measures and parameters of iron status were determined in venous blood. RBV, blood volume (BV), and plasma volume (PV) were measured by CO rebreathing. For determination of the aerobic and anaerobic capacity (maximal accumulated oxygen deficit, MAOD), the athletes performed an incremental as well as a highly intensive treadmill test. After 12 wk, ferritin levels were within the normal range in the iron-treated group (IG) with a significant (P < 0.001) mean increase by 20 microg.L-1 opposed to a slight nonsignificant decrease in the placebo group (PG). RBV did not change significantly in either group nor did any of the hematological measures. However, only in IG there were significant increases in VO2max and in O2 consumption in the MAOD test. MAOD and maximal capillary lactate concentration remained unchanged in both treatment groups. The results indicate that in young elite athletes with low serum ferritin and normal hemoglobin concentration iron supplementation leads to an increase in maximal aerobic performance capacity without an augmentation of RBV.