The aim of this study was to evaluate the lean body mass variation in obese children and adolescents under Protein Sparing Modified Fast (PSMF) treatment through two indirect, not invasive, methods: The Slaughter-Lohman formal as an anthropometric method, and the urinary excretion of 3-Methylhistidine (3MHIS) as a biochemical method. Materials and methods. Thirty-five subjects, divided in two groups, participated in this study: Group A, 12 children (4 girls, 8 boys, age 9.5±1.9, OW% 73.8±20.1); Group B, 19 adolescents (14 girls, 5 boys, age 14.6±2.3, OW% 61.7±19.7). Diet: PSMF: Protein g 1.8±0.08 kg. IBW. day, total Kcalories 10.64±0.63 kg. IBW.day, prescribed for 9 weeks ± 1. Results. The 3MHIS final values were not significantly increased compared to the initial ones in both groups. In both groups the final value of lean body mass was significantly higher than the initial ones (p<0.03) in terms of percentage and Kg, while the fat mass was significantly reduced. Conclusion. With all the limitations using indirect methods to measure body composition, it seems that a PSMF, prescribed for 9 weeks ± 1, does not cause a significant lean body mass loss in obese children and adolescents.