Anaerobic threshold (AT) has been advocated as an objective method of evaluating exercise capacity in patients with chronic congestive heart failure. The factors that determine AT, however, remain still unclear. To assess the influence of oxygen transport capacity on AT, patients with iron deficiency anemia were studied before and after treatment with iron. Twenty-nine female subjects were studied. They were divided into the following 3 groups: 1) iron deficiency anemia (group IDA: Hgb less than 11 g/dl and ferritin less than 10 ng/ml) consisting of 4 athletes and 6 non-athletes, 2) latent iron deficiency (group Lat-ID: Hgb greater than or equal to 11 g/dl and ferritin less than 10 ng/ml) consisting of 4 athletes, and normal (group Nor: Hgb greater than or equal to 11 g/dl and ferritin greater than or equal to 10 ng/ml) consisting of 15 athletes and 6 non-athletes. By bicycle ergometer using ramp protocol, peak oxygen uptake (peak VO2) and AT were measured in each group. Following the 1st exercise testing, groups IDA and Lat-ID were treated by oral iron for 1-1.5 months. The 2nd exercise testing was then performed. Furthermore, to investigate whether muscle cell energy metabolism itself is altered by iron deficiency, P magnetic resonance spectroscopy (MRS) was performed in 2 relatively severe anemic patients during forearm exercise while assessing the changes in phosphocreatine and inorganic phosphate. Peak VO2 and AT in non-athletes were significantly lower in IDA group than Nor group (peak VO2 (ml/min/kg): 23.7 +/- 5.1 vs 33.3 +/- 3.8, p less than 0.01, AT (ml/min/kg): 15.9 +/- 3.3 vs 21.3 +/- 1.3, p less than 0.01). After iron administration, Hgb was increased significantly in IDA group (from 9.0 +/- 1.8 to 12.1 +/- 0.8 g/dl, p less than 0.01) accompanied by an improvement in peak VO2 and AT (peak VO2 (ml/min/kg): from 34.2 +/- 12.4 to 40.0 + 13.0, p less than 0.001, AT (ml/min/kg): from 20.9 +/- 6.3 to 25.0 +/- 8.0, p less than 0.001). Lat-ID and Nor groups showed no changes. MRS indices of cell energy metabolism of the 2 severely anemic patients did not differ from those of normal controls, and no changes were observed after iron treatment. It is concluded from these results in iron deficiency anemia that oxygen transport is a determinant of anaerobic threshold.