Arm span and stature were measured at 6-month intervals in 160 normal children from 8 to 15 years of age. The mean difference between arm span and stature is greater in boys than in girls at all ages. In boys, the mean arm span is always greater than mean stature. The difference in girls in always less than 1 cm, but the sign changes from negative to positive at 11.5 years. An arm span-stature difference outside the 90% confidence limits suggests the possibility of a clinical condition that differentially affects the elongation of the trunk and of the limbs.