Isokinetic strength and cross-sectional area (CSA) of knee extensor muscles were examined in 60 boys and girls, aged 6-9 years, and in 71 young adult men and women to investigate the influences of age and gender on the strength capabilities in relation to muscle size. A single anatomical CSA of quadriceps femoris at the right mid-thigh was measured by using a B-mode ultrasonic apparatus. Isokinetic strength produced at three kinds of constant velocity, 1.05, 3.14, and 5.24 rad.s-1, was significantly correlated to the product of CSA and thigh length (TL) in separate samples by age and gender. In both genders, however, young adults had significantly higher ratios of strength to CSA.TL (ST.CSA-1.TL-1) than children at all measurement velocities. Relative differences in ST.CSA-1.TL-1 between children and young adults became larger with increasing test velocity. Within the same generation, men had significantly higher ST.CSA-1.TL-1 than women at all measurement velocities except for the ratio in children at 1.05 rad.s-1. These results indicate that the ability to produce strength proportional to muscle size is lower in children than in young adults, regardless of gender, and lower in women than in men within the same generation.