The aim of this study was to determine the relative importance of lean mass (LM) and fat mass (FM) on bone mineral density (BMD) in a group of adolescent girls and boys. A total of 65 adolescent boys and 35 adolescent girls participated in this study. Whole body (WB) and lumbar spine (L1-L4) BMD were measured by dual-energy X-ray absorptiometry (DXA). Body composition was assessed using the same technique. In boys, LM was strongly related to WBBMD (r = 0.68; p < 0.001) and to L1-L4 BMD (r = 0.61; p < 0.001), whereas FM was not positively related to BMD and was negatively associated with WB bone mineral apparent density (WBBMAD). In girls, both LM and FM were positively related to WBBMD (r = 0.41; p < 0.05 and r = 0.49; p < 0.01, respectively), whereas only FM was correlated to L1-L4 BMD (r = 0.33; p < 0.05). Finally, in a multiple regression analysis, FM was found to be a better positive determinant of WBBMD than LM in girls, whereas in boys, FM was found to be a negative determinant of WBBMD and L1-L4 BMD. This study suggests that LM is a strong determinant of WBBMD and L1-L4 BMD in boys, and that FM is a stronger determinant of WBBMD than LM in girls.