Aim: To assess body mass index. (BMI) in relation to sexual maturation and verify how fat-free mass (FFM) and fat mass (FM) contribute to its increase. Methods: 1,275 White adolescents (656 girls and 619 boys), aged 9 to 18 years, were evaluated according to Tanner stages. FFM and FM were calculated by triceps and subscapular skinfold measurements and leg-to-leg bioelectrical impedance. BMI-for-maturation was distributed into percentiles. Fat-free mass index (FFMI) and body fat mass index (BFMI) were calculated based on BMI 50(th) percentile values. Results: During maturation, as BMI increases, girls gain FFMI (13.3 kg/m(2) at 131 and 15.8 kg/m(2) at 135) and BFMI (3.7 kg/m(2) at B1 and 5.7 kg/m(2) at B5) - proportionally more fat - while boys gain FFMI (14.3 kg/m(2) at G1 and 17.9 kg/m(2) at G5) and lose BFMI (4.8 kg/m(2) at G1 and 3.2 kg/m(2) at G5). Conclusion: These data suggest that FFMI and BFMI-for-maturation should be assessed in adolescents monitored for overweight and obesity.o TEXTO COMPLETO DESTE ARTIGO, ESTARÁ DISPONÍVEL À PARTIR DE AGOSTO DE 2015.212127133