BACKGROUND Current findings suggest that more attention needs to be given to the increase in body mass achieved by disabled populations, especially by individuals with mental retardation, to minimize long-term negative health consequences. Accordingly, it would be of interest to design adequate strategies based on physical activities that may be easily performed to ensure adherence as a healthy lifestyle choice for these populations. MATERIAL/METHODS To attain this goal, 22 male adolescents with Down's syndrome (mean age: 16.2 +/- 1.0 years) underwent a 12-week physical exercise intervention consisting of three sessions of one hour per week in both water and on land for 12 weeks. Fat mass percentage was calculated from anthropometric measurements according to the Durnin-Womersley equation. A paired t test was performed to evaluate possible differences in antropometrical characteristics between before and after the physical exercise intervention. RESULTS According to the body mass index, it was observed that 31.8% of the studied individuals presented overweight and 27.3% of them were obese before starting our experiment. The mean value of the percentage of fat mass was reduced significantly, from 31.8 +/- 3.7% to 26 +/- 2.3%, at the end of the study (p = 0.021). CONCLUSIONS We may conclude that the adolescents with Down's syndrome were able to reduce their fat mass percentage significantly when performing a 12-week training program, which could have important impact on the comorbidity associated with obesity and on the quality of life of this population.