Professional tennis players have a marked inter-arm symmetry due to 10-20% greater muscle mass in the dominant compared to the non-dominant arm. However, the effect of tennis on individual muscles has not been previously studied. PURPOSE: The aim of this study was to describe the effect of professional tennis participation on the individual muscle volumes of the dominant arm, and the changes elicited in the morphology of the triceps brachii muscle, using the non-dominant arm as a control. METHODS: Fifteen professional tennis players (22±4 years) were divided into two groups: one underwent muscle volumes examination by magnetic resonance imaging (n=8) in both arms, and the other group a body composition examination (DXA, n=7) and muscle biopsies from both triceps brachii (short head, n=7). RESULTS: Muscle fiber type distribution was similar in both arms. The dominant arm had 15% higher lean mass (DXA) than the non-dominant arm (P<0.05).The type 1, 2a and 2x muscle fibers of the dominant arm were hypertrophied compared to the non-dominant arm by 20 (4962 ± 452 and 4210 ± 460 μm2, P < 0.01), 22 (7700 ± 873 and 6311 ± 707 μm2, P < 0.01) and 34% (7058 ± 878 and 5225 ± 451 μm2, P < 0.01), respectively. Compared to the contralateral arm, the dominant arm had a 10% greater mean muscle volume (P < 0.001). All the muscle groups of the dominant arm were hypertrophied compared to the non-dominant arm: the deltoid muscle 13 % (P < 0.001), the arm flexors 9 % (P < 0.05) and the triceps brachii 13 % (P < 0.05). The volume of the superficial flexor muscles of the forearm was 12 % higher in the dominant compared to the contralateral forearm (P < 0.05). No significant differences were observed between the dominant and non-dominant forearm in deep flexors, mobile pad, forearm extensors and supinator muscle volumes (P = 0.62, P = 0.23, P = 0.98 and P = 0.59, respectively). CONCLUSIONS: Professional participation in tennis elicits an harmonic muscle hypertrophy in the dominant arm, where individual muscle maintain between them the same proportions observed in the contralateral arm. However, the forearm muscles show great variability between tennis players. Granted by (BFI2003-09638, BFU2006-13784, PI2005/177 and FEDER). The authors wish to thank Hospital San Roque Maspalomas (Grupo San Roque) and Sánchez-Casal Tennis Academy.