The present study aimed to examine the influence of lateral dominance for ball kicking on the cross-sectional areas (CSAs) of thigh and trunk muscles in Japanese elite youth and professional soccer players, and to clarify the difference between the 2 groups in the muscle CSAs of the 2 body segments in relation to that in lean body mass (LBM). The CSAs of 4 (rectus abdominis, oblique, psoas major, and erector spinae) and 3 (quadriceps femoris, hamstrings, and adductors) muscle groups located in the trunk and thigh, respectively, were determined in 18 youth players (16.8+/-0.6 years) and 17 professional players (23.7+/-3.1 years) using magnetic resonance imaging. In youth and professional players, no significant effect of lateral dominance was found in the CSA of any muscle group. In all muscle groups except for the erector spinae, the CSAs were significantly greater in the professional players than in the youth players. The CSA of every muscle group was significantly correlated to the two-thirds power of LBM (LBM). In terms of the ratio of CSA to LBM, only the psoas major was significantly greater in the professionals. In conclusion, Japanese youth and professional soccer players did not exhibit bilateral asymmetry in the CSAs of thigh and trunk muscles, and the professional players had more developed psoas major muscle as compared with youth players even when matched for whole-body lean tissue mass. The current results suggest that for soccer players with bilateral asymmetry in the muscularity of the thighs and trunk, personalized strength programs for developing symmetry are recommended, and exercises involving hip flexion should be incorporated progressively into individual strength and conditioning programs.