Humeral head retroversion is important in a variety of clinical situations, but it is not known when retroversion actually develops to adult values. In utero and at birth, the humeral head is known to be in marked retroversion. It derotates sometime thereafter to assume the more standard value with which orthopedic surgeons are familiar. By studying a unique collection of children's bones (180 humeri), I have been able to determine that the greater part of this process takes place, on average, by the age of 8 years (SD +/- 2.12 years); from then on, however, the development continues more slowly until the final adult dimensions are reached, as heralded by the appearance of the radial groove at approximately 16 years of age. This process, along with its timing, appears to be similar, but opposite in direction to, the derotation process that is known to take place in the hip. It has not previously been documented in the shoulder.