More than ever young athletes are training intensively and compete at high levels. As adolescent athletes are training in a period associated with many changes in growth and maturation longitudinal studies are important to elucidate the development in physical parameters. The aim of this study was to evaluate the development in aerobic power in elite as well as non-elite soccer players during the years of puberty and to relate it to maturation as well as hemoglobin and hematocrit.