Factors that determine oxygen uptake during stepping exercise (modified Canadian Aerobic Fitness Test, CAFT) were examined in 66 women and 55 men. Subjects ranged in age from 15 to 67 yr, VO2max from 22.5 to 76.9, leg length from 75.0 to 101.0 cm, and body mass from 48.4 to 107.7 kg. In accordance with the modified CAFT protocol, subjects stepped at cadences determined by their age and heart rate response. The oxygen demand of stepping at each cadence was measured on two occasions. A paired t-test revealed no significant (P > 0.05) difference in oxygen demand between the repeats and the intraclass correlation coefficient was 0.51. The coefficient of variation of oxygen uptake (ml O2.kg-1.min-1) at a given stepping cadence averaged 10.4%. Regression analysis indicated that little variation in oxygen demand could be explained by simple descriptors of the subjects. Age was weakly related to oxygen demand at a given level (males r = 0.26, females r = 0.58). Stepwise multiple regression of the oxygen uptake at selected stepping cadences on possible independent variables confirmed that little of the variation could be explained by age, aerobic fitness, leg length, or adiposity (maximum R2 = 0.34). We conclude that variation in the oxygen demand of a stepping task can account for a large portion of the error in predicting VO2max from a submaximal stepping test. Our ability to predict the oxygen demand of stepping from subject characteristics is limited.