The purpose of this study was to investigate the development in muscle cross-sectional area (CSA) and strength capability of the reciprocal muscle groups in the upper arm and thigh. Subjects were 130 boys aged 7 to 18 years; 7-9 yr aged-group (n = 30), 10-12 yr (n = 30), 13-15 yr (n = 35), and 16-18 yr (n = 35). Anatomical CSAs of elbow flexors (EF), elbow extensors (EE), knee flexors (KF) and knee extensors (KE) were determined with a B-mode ultrasound apparatus. Isokinetic strength during elbow or knee extension as well as elbow or knee flexion was measured by using a Cybex II isokinetic dynamometer at the pre-set constant velocity of 60 degrees/s. The effect of age associated with a significant increase in both CSA and strength with a marked increase in 13-15 yr. The elder aged-groups had a significantly higher ratio of strength to muscle CSA than in the lower aged-groups. The effect of age on EF/EE CSA ratio was not significant, but EF/EE strength ratio was the highest in 16-18 yr and the lowest in 7-9 yr. With advancing age, KF/KE CSA ratio had a tendency to increase, but KF/KE strength ratio remained almost unchanged. These results indicate that (1) children in the pre-puberty or the early stage of puberty do not develop strength in proportion to their muscle CSA, and (2) the flexors and extensors in limbs have reciprocally different growth rates in either CSA or strength even in the same region.