Induced erythrocythemia is associated with a variable increase in maximal oxygen uptake. To examine the roles of the magnitude of change in hemoglobin concentration and the individual's initial aerobic fitness in this effect, we combined individual data from our own research and three other studies. In each study freeze-preserved erythrocytes from the product of 2 units of blood were reinfused, and maximal oxygen uptake was measured within 24 to 72 hours after reinfusion. The 30 subjects had an initial aerobic power of 36 to 88 mL of oxygen per kilogram per minute. The combined results from these studies indicate that after erythrocyte reinfusion (1) the increase in hemoglobin concentration is fairly homogeneous (mean ± SD, 1.36 ± 0.6 g/dL [13.6 ± 6 g/L]), (2) nearly all individuals demonstrate an increase in maximal oxygen uptake (0.357 ± 0.216 L/min), (3) the magnitude of increase in hemoglobin concentration is not related to the magnitude of increase in maximal oxygen uptake, and (4) the magnitude of increase in maximal oxygen uptake is related to the individual's initial aerobic fitness. Individuals with an initial aerobic fitness between 50 and 65 mL·kg-1·min-1 experience approximately twice the increase in maximal oxygen uptake after erythrocyte reinfusion of individuals with greater fitness and also of lesser fitness.