[en] A case of necrosis of the femoral head due to radiation therapy of the hip joint in early infancy is presented. The patient (a seven-year-old female) who had been diagnosed as malignant melanoma four months after birth, received radiation therapy of 6500 R by a betatron. She also received radiation therapy of 8100 R after the radical operation. When she was two years old, a diagnosis of necrosis of the femoral head was made. When she was four, she had extreme lameness of the right leg. Subtrochanteric osteotomy was performed at the diseased site of the right hip joint, and a patho-histological study was made of a biopsy. The authors also report the microangiographical and patho-histological studies of fifty young rabbits which had received 300 rads and 6000 rads of Linac X-ray. (Ichikawa, K.)