Changes in height and weight during childhood and adolescence are well documented, yet there is comparatively little comprehensive information about muscular development during this time. In a cross-sectional survey standing height, body weight and isometric strength of the elbow flexor and quadriceps muscles have been measured in 267 boys and 284 girls aged from 5 to 17 years. All the children were from private London schools. The mean heights and weights for each age group were between the 50th and 75th centiles for British children. The strength of both muscle groups in the boys and girls rose steadily in each age group from 8 to 12 years, after which there was a rapid increase in strength of both the quadriceps and elbow flexors in boys which continued even when growth in height and body weight had virtually ceased. In the pre-adolescent phase of growth, muscle strength of the elbow flexors and quadriceps increased as a function of height squared and cubed respectively, suggesting that stretch as a result of elongation of the long bones, and for the quadriceps, loading, may be the primary stimuli during this phase. In the postpubertal phase some other stimulus, such as a direct action of hormones on the muscle, must be responsible for the continued increase in strength in the boys.