Body mass (BM) and composition are considered to be significant determinants of aerobic endurance. The purpose of this study was to examine the influence of BM and its composition makeup on endurance performance. Thirty-nine men were divided into 3 groups of significantly different BM and mass composition: a control group, a group with high body fat (HBF), and a group with high lean body mass (HLBM). Participants in the HBF and HLBM groups had similar BM, which was significantly greater than observed in the control group. All participants performed an incremental treadmill test to volitional failure. Endurance performance was assessed on the basis of respiratory compensation point (RCP). The HBF and HLBM groups exceeded RCP at a much lower work intensity, expressed as %HRmax and %VO2max, compared with the control group. When considered relative to BM and lean body mass, oxygen consumption values at RCP were significantly less in the HBF and HLBM groups compared with the control group. Increased BM, regardless of its composition, substantially reduced aerobic endurance performance. Therefore, athletes and coaches should pay attention to not only the percentage of body fat but also to the amount of lean body mass because any gain may adversely affect aerobic endurance performance.