Objective. - The aim of this study is to analyze the effects of nine months of martial arts practice (Judo and Muay Thai) on the aerobic fitness of children and adolescents. Methods. - The sample consisted of 64 young people (41 boys and 23 girls) participating in a social project, aged between 6 and 15 years (9.06 +/- 2.11). The children and adolescents were divided into three groups: Control (n = 26), Judo (n = 21), and Muay Thai (n = 17). Anthropometric measurements were performed, and the body mass index and somatic maturation were calculated. Maximum oxygen uptake (one-mile run/walk test) and agility (Shuttle Run) were measured. The intervention was performed for nine months, twice a week, for 60 minutes. Both groups (Judo and Muay Thai) performed 20 minutes of general exercises, 20 minutes of specific exercises for each modality, and 20 minutes of fight simulation and/or play activities. Results. - At baseline the groups were similar for all characteristics and aerobic fitness. After 9 months, the MT (Delta= 1.71 +/- 0.74) and Judo (Delta= 1.25 +/- 0.68) groups presented increased oxygen consumption compared to the Control group (Delta= -1.85 +/- 0.61), which presented decreased values (p-value = 0.001). Agility, however, did not present a difference between the groups (Control Delta= -0.52 +/- 0.24, Judo Delta= -0.74 +/- 0.27 and MT -1.08 +/- 0.29). Conclusion. - Nine months of martial arts training increased maximal oxygen consumption. (c) 2022 Published by Elsevier Masson SAS.