Twenty-two trained women runners (.VO2peak 48.1 + 1.2 ml x kg -1 x min -1) were divided into an iron supplement (n = 13) or placebo group (n = 9) based on initial serum ferritin concentration (24.2 +/- 2.9 and 58.5 +/- 4.0 microg x l -1, respectively). Exercise consisted of a 35-min run (80 % .VO2peak) and was performed at week 0 (WK0), after two weeks of intensified training (WK2) and after eight weeks recovery training (WK10). The eight weeks recovery training were concomitant with subjects taking iron supplements or placebo in a double blind fashion. Concentrations of serum ferritin, serum iron and total iron binding capacity were assessed pre-exercise and complete blood count, natural killer cell activity (NKACT), and cell surface markers for CD3+, CD4+, CD3+,CD8+, CD3-, CD16+, CD56+ cells were determined both pre- and post-exercise. Serum ferritin concentrations were significantly (p < 0.05) increased on WK10 compared to WK2 (time effect). NKACT (%lysis) and NK cell number was lower (p < 0.05) at WK0 for supplement (42.9 +/- 1.9 % and 305.5 +/- 15.0 x 10 6 x l -1, respectively) compared to placebo groups (50.9 +/- 2.0 and 406.1 +/- 25.6, respectively). Two weeks of intensified training did not alter indices of host defense. In conclusion, NKACT and NK cell numbers were lower in subjects with greater body mass and lower iron stores (p < 0.05), but were not significantly altered after two weeks of intensified training or when serum ferritin levels increased.