Results of a study of the effects of external irradiation on growing bone conducted at Stanford University School of Medicine are presented together with a review of the literature. Standing and sitting heights of 29 children receiving more than 3,500 rads of megavoltage radiation to the spine and 15 children receiving less than 2,500 rads were compared with those of 15,000 normal children. Retardation of spinal growth was seen in children irradiated during the periods of most active bone growth, i.e., under 6 years of age and during puberty. Correlative radiographic findings similar to those seen with orthovoltage therapy were seen in the high-dose group but not in the low-dose group.