Introduction. Tennis is a sport that requires asymmetric movements. Asymmetry in tennis pertains to the player carrying a racket and using it to hit the ball. An asymmetric tennis technique may lead to an asymmetric distribution of muscle mass and unbalanced muscle tonus. These disproportions will result in an improper body stature and may even cause irregularities in the skeletal structure. Aim of the Study. The aim of this study was to determine the degree of muscle mass asymmetry and its association with the dominant upper limb. Material and Methods. The study included 16 active tennis players: 15 right-handed and 1 left-handed. The control group (UN) comprised 16 non-training middle school pupils: 14 right-handed and 2 left-handed. Body composition was determined by means of bioelectrical impedance analysis (BIA). Results. Significant differences between the dominant and non-dominant arms in tennis players were observed. The players featured a higher muscle mass of the dominant upper limb compared to the non-dominant limb. Similar differences were not observed amongst the controls. The control group was characterized with markedly lower asymmetry than the tennis players in terms of muscle mass distribution in the upper limbs (p < 0.05). No significant correlations were found between age or training experience and the asymmetry coefficient values. Conclusions. The study revealed a significant influence of sport training on asymmetry in muscle mass distribution in the upper limbs.