The authors investigated the association of the amount and intensity of conditioning leisure time physical activity with serum ferritin and blood hemoglobin concentrations in 1,743 eastern Finnish men who were aged 42-60 years during the period 1984-1989. The duration and frequency of physical activity were associated inversely with serum ferritin (p = 0.003 for duration and p < 0.001 for frequency) and blood hemoglobin (p = 0.002 for duration and p = 0.019 for frequency) in multivariate regression models, after adjustment for major confounders. Men in the highest quartile of duration (> 2.6 hours/week) had a 16.8% lower mean serum ferritin concentration and men in the highest category of frequency (> 3 sessions/week) had a 19.9% lower mean serum ferritin concentration than men with a low duration (< 0.4 hour/week) and frequency (< 1 session/week), respectively. For blood hemoglobin, the respective differences were 1.3% and 1.0%. The intensity of physical activity was significantly associated only with blood hemoglobin (p = 0.011). Together with the authors' previous finding concerning the association between high serum ferritin and an excess risk of acute myocardial infarction, these data suggest that a reduction in stored iron levels could be one mechanism through which conditioning leisure time physical activity decreases the risk of coronary heart disease.