Mortality up to 1 January 1983 has been studied in 14,106 patients with ankylosing spondylitis given a single course of X-ray treatment during 1935-54. For neoplasms other than leukaemia or colon cancer, mortality was 28% greater than that of members of the general population of England and Wales, and this increase is likely to have been a direct consequence of the treatment. The proportional increase reached a maximum of 71% between 10.0 and 12.4 years after irradiation and then declined. There was only a 7% increase in mortality from these tumours more than 25.0 years after irradiation and only for cancer of the oesophagus was the relative risk significantly raised in this period. Neither the magnitude of the relative risk, nor its temporal pattern following treatment, were greatly influenced by the age of the patient at first treatment. For leukaemia there was a threefold increase in mortality that is also likely to have been due to the radiotherapy. The relative risk was at its highest between 2.5 and 4.9 years after the treatment and then declined, but the increase did not disappear completely, and the risk was still nearly twice that of the general population more than 25.0 years after treatment. There was some evidence that the risks of acute myeloid, acute lymphatic, and chronic myeloid leukaemia were all increased, but no evidence of any increase in chronic lymphatic leukaemia. The relative risk appeared to be greatest for acute myeloid leukaemia. For colon cancer, which is associated with spondylitis through a common association with ulcerative colitis, mortality was increased by 30%. For non-neoplastic conditions there was a 51% increase in mortality that was likely to be associated with the disease itself rather than its treatment. The increase was apparent for a wide range of diseases and was not confined to diseases that have been associated clinically with ankylosing spondylitis.