Among the unfortunate late results of x-ray and radium therapy are the deformities which result from disturbances in the longitudinal growth of irradiated long bones of children. This effect of roentgen rays was demonstrated experimentally by Perthes (1) soon after these rays had been discovered, and since then there have accumulated several reports both of illustrative clinical cases and of comprehensive experimental investigations. The most complete and recent study was reported by Brooks and Hill-strom (2), in 1933. Although many observations herein recorded merely repeat and confirm those previously reported, the clinical importance of the subject and the apparent general lack of appreciation of its importance justify emphasis by repetition. Case Reports Case 1. A. J., a girl five years of age, was seen in consultation by Dr. Hunt. Her parents were perturbed regarding the failure of her right index finger to grow. Two years previously, radium had been applied by another physician over a nevus on the dorsum of the finger, with no record of dosage. The nevus disappeared, but atrophy and telangiectasia gradually appeared. The finger failed to lengthen, eventually being shorter than the little one. As illustrated in Figure 1-A, the right index finger was much shortened, and the skin presented the atrophic and vascular changes which result from intensive irradiation, relatively rich in beta rays. The roentgenogram, Figure 1-B, shows that all epiphyseal lines of this finger except the proximal one of the proximal phalanx have closed. Thus, growth from all but one source has been irrevocably terminated. Case 2. Q. M., a boy 17 years of age, presented a genu varum deformity, with 2.5 inches shortening of the left leg. He gave a history that 12 years previously a tumor on the mesial aspect oi the knee had been irradiated with both radium and x-rays. There developed a varus deformity and shortening, which progressed until two years ago (Figs. 2-A and 2-B), when the bowing and some of the shortening were corrected by a supracondylar osteotomy of the femur (Fig. 2-C). Since then some of the bowing has recurred. In addition to the slight deformity and a shortening of 2.5 inches, there is a large, soft tissue defect covered by a thin telangiectatic scar. Roentgenograms taken two years ago show that the epiphyseal line had closed on the mesial aspect but was still present on the lateral side (Fig. 2-B). Growth, continuing on the lateral side but not on the mesial side, had thus twisted the lower end of the femur into a varus deformity. Experimental Studies Five litters of rabbits from three to five weeks old were used. In each animal one extremity was irradiated and the opposite one served as a control.