Abstract:Background: Percent body fat, fat mass and fat mass index are measures of body fat while fat free mass and fat free mass index measure lean mass. These measures of body composition vary with age and sex. Aim: To show the age-related patterns of measures of fat and lean mass in adolescent Nigerians in Lagos. Methods: Percent body fat was measured in school pupils aged 10years to 18years (377 boys and 376 girls) using Tanita ® body fat monitor (BF 666). Fat mass (FM) was derived from percentage body fat and body mass and fat free mass (FFM) was obtained by subtracting fat mass from body mass. Fat mass index (FMI) and fat free mass index (FFMI) were derived by dividing FM and FFM, respectively by the square of the height. Results: Overall percent body fat and FM were significantly higher in girls (18.9±7.5% Vs 9.5±4.5%: p < 0.001 and 9.7±6.1kg Vs 4.5±3.2kg: p < 0.001 respectively). FMI for girls ranged from 3.2 to 4.5kg/m² peaking at 16 years. At all ages, girls had higher mean FMI than boys. The mean FMI for males fell from 2.6 kg/m² at 10 years to a trough of 1.5 kg/m² at 16 years before a slight rise to 1.9 kg/ m² at 18 years. FFM in boys increased consistently with age, overtaking that of girls at 12 years with the gap widening up to 18 years. Measures of body fat were much lower in study subjects than reported from western countries even where lean mass was comparable. Conclusion: Adolescent females have higher body fat indices while males have higher lean mass indices. Indices of body fat in the current study are much lower than reported for western counterparts.