To observe the cross-sectional nature of the effect of age, height, and body mass on motor performance during adolescence (13-18 years), 103 boy and 65 girl athletes were measured for motor performance and anthropometric variables. Motor performances included tests of strength, muscular endurance, flexibility, aerobic capacity, anaerobic power, speed, and agility. Anthropometric determinations included height, body mass, lean body mass, %fat, and somatotype. Boys were significantly different from girls in all measurements except endomorphy, while girls were significantly superior to boys only in flexibility. Physical maturation, as reflected by height and body mass, was a major contributor to increases in motor performance. Somatotype did not differ greatly across the age groups. Boys were significantly more mesomorphic than girls, while girls were significantly more ectomorphic than boys. Higher %fat and more endomorphy were significantly related to poorer performance for relative aerobic capacity, 40-yd dash, and agility in boys but only for upper body muscular endurance in girls. Mesomorphy had higher relationships with performance variables among boys than among girls. Growth would appear to contribute significantly to enhanced motor performance with age, and its effect may be different in boys than in girls.