The oncogenic effects of childhood irradiation of the thymus gland were investigated by following up 466 irradiated subjects, 506 unirradiated subjects with similar illnesses, and the siblings of both series. In the irradiated group there were two malignant and nine benign thyroid neoplasms, indicating an excess risk in this group. There was also a suggestion of an increased risk of breast cancer. There was no evidence of difference between the hospital and sibling controls; this supports the validity of previous studies using sibling controls.