The objectives of this work were: a) to analyze the relationship between anthropometric variables, chronological age, somatic maturation and maximum expiratory flow with Fat Free Mass (MLG) of young swimmers and b) propose regression equations that allow to predict MLG of swimmers by biological age and sex. We studied 36 swimmers (21 men and 15 women) between 10.0 and 16.9 years old, from the selection of Maule Swimming (Chile). Weight, height, sitting height, forearm length, calf circumference and Maximum Expiratory Flow (FEM) were evaluated. Chronological age and biological age were calculated by anthropometry. The values of body composition (MLG, Bone Mass and fat mass) were obtained by means of Dual X-ray Absorptiometry (DXA). MLG was related to chronological age (R2 = 0.46), biological age (R2 = 0.69), weight (R2 = 0.86), height (R2 = 0.77), sitting height (R2 = 0.66) forearm length (R2 = 0.69), circumference of the right arm (R2 = 0.55) and FEM (R2 = 0.69). Four regression equations were generated for each sex, which showed an adjusted R2 between 86% and 90% in men, and in women between 95% and 97%. The biological age, the length of the forearm, the circumference of the calf and the FEM are excellent predictors of MLG. The generated equations serve to control, diagnose, and monitor the MLG of young swimmers by biological age and sex.