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Measures of body density and radiographs of the left knee were collected from 30 boys and 39 girls, 10 to 18 years old, at 2 or 3 consecutive annual examinations, and annual increments of lean body mass, and total and percent body fat were calculated for each participant. Annual increments of lean body mass were negatively associated with chronological and skeletal age of the knee in girls. Other age or maturational effects upon these variables were absent in each sex. Between 10 and 18 years of age, annual increments for lean body mass in girls decrease with age, but not in boys who have a mean annual increment in lean body mass across age of 4.38 kg/yr. For girls, there is little or no change in percent body fat between 10 and 18 years, but boys have a mean annual decrease across age of 1.15 %/yr. For the boys, there is no significant change in total body fat between 10 and 18 years, but girls have a mean annual increase in total body fat across age of 1.14 kg/yr.