The purpose of the present study was to determine the effect of isometric strength training in prepubescent boys and girls on muscle strength and cross-sectional area of upper arm muscle. Subjects were ninety-nine healthy elementary school children from the 1 st, 3 rd and 5th grades and who were assigned to either training (TG, n=52) or control (CG, n=47) groups. The TG participated in a strength training program for 12 weeks which consisted of three maximaly sustained isometric contractions of elbow flexion for ten sec, twice a day, three days per week, whereas the CG did not participate in a special training program during this period. The cross-sectional areas of whole tissue, muscle, fat, and bone of the upper arm were measured by ultrasonic methods. Maximum isometric and isokinetic muscle strength of elbow flexion and extension were measured by means of a Cybex II. The hand-wrist X-ray photograph was used to estimate the skeletal age (TW 2 method) . Means (SDs) of skeletal age (yrs) were 6.27 (0.98), 8.48 (0.89), and 10.77 (1.22) for 1 st, 3 rd, and 5 th graders, respectively. After the 12-week training period, the cross-sectional area of the whole tissue increased significantly in both TG (8.9%, 2.52 cm²) and CG (7.3%, 2.11 cm²) . This interment in the TG was due to the significant increases in both muscle and bone areas, while that in CG was due to the significant increase only in fat area. Mean increase in muscle area in the TG was 10.3% (1.29 cm²) with this increase notable in 5th graders (12.5% for males and 12.7% for females) . The increment in cross-sectional area of muscle with training significantly correlated with the skeletal age (r=0.36, p<0.01) . Maximum isometric strength in the TG increased by 5.7% (0.3kg) in flexion and 17.5% (1.2 kg) in extension and in the CG, -0.3 kg (-1.0%) in flexion and 5.7% (0.3 kg) in extension. The increases in the strength of elbow extension were statistically significant in 1st (p<0.01), 3rd (p<0.05), and 5th (p<0.05) grade males. On the other hand, maximum isokinetic strength did not change with training. Muscle strength per cross-sectional area did not show a significant increase except in the training group of 5th grade boys. It was suggested that the effects of strength training on muscle area and strength for prepubescents were similar in its direction to but different in its magnitude from those found in adults.