Élite adult athletes are known to have physical and physiological characteristics specifically suited to their sport. However, it is not clear whether the observed adult differences arise because of training or whether the sport selects the individual with the appropriate characteristics.The purpose of this study was to compare and contrast the physical development of young athletes (8 - 19 yr) and in so doing provide an answer to this question. Development of anthropometric characteristics, sexual maturation, pulmonary function and aerobic power were assessed in a group of 232 boys and 222 girls. The athletes were a randomly selected group of young athletes who had demonstrated previous performance success or who were excepted to do so in the future. They came from 4 sports namely: soccer (all male); gymnastics (2:1 female to male ratio), swimming (1:1 sex ratio) and tennis (1:1 sex ratio). The subjects were assessed annually for three consecutive years. The adjusted mean (ANCOVA) height of male swimmers (161.6 ± 0.6 cm) was found to be significantly greater (P<0.01) than non-athletes (159.2 ± 0.4 cm), gymnasts (150.7 ± 0.8 cm) and soccer players (158.7 ± 0.6 cm) and adjusted body mass (51.3 ± 0.6 kg) significantly greater (P<0.01) than the other groups. These trends were also observed in females. When testicular volumes were compared, it was found that swimmers matured significantly earlier (P<0.05) than gymnasts, tennis players and a reference population of non athletes. Female gymnasts attained sexual maturation (indexed by menarche) on average (14.4 ± 0.2 yr) a year after the other sports and the general population. A positive correlation was found between menarcheal age in mothers and daughters (r=0.29 , P<0.05), suggesting a familial trait. The observed late sexual maturation of gymnast therefore suggests some form of sports specific selection. Swimmers had the highest initial lung volumes (P<0.001), a difference which did not change with time. However, as training began well before the subjects were tested it was not possible to determine whether these observed differences were present prior to training. When age, height and weight were controlled for VO₂ max in males significantly increased both pubertal development, although this pattern was not shown in females. Swimmers had the highest VO₂ max values at all ages.