The relationship between exercise and energy expenditure is unclear, with some suggestions that exercise leads to increased energy expenditure over and beyond the increase due to the exercise itself. In this cross-sectional study, we examined the relationships among aerobic fitness level, body composition, and total daily energy expenditure in 78 subjects. Daily energy expenditure (determined in a whole room calorimeter) was significantly correlated with both fat-free mass (FFM) and aerobic fitness (estimated from maximum aerobic capacity or VO2max). However, multiple-regression analysis demonstrated that, after accounting for FFM, VO2max did not explain a significant amount of the remaining variation in energy expenditure. In addition, the relationship between resting metabolic rate and both FFM and VO2max was evaluated using data from 214 weight-stable subjects analyzed retrospectively. The results were identical with the results obtained from the 78 subjects in that VO2max did not have effects independent of FFM on energy expenditure. We conclude that aerobic fitness does not have a direct effect on energy expenditure. However, it may have effects that are mediated through body composition, since in both populations studied here, VO2max was positively correlated with FFM and negatively correlated with adiposity.