Between January 1949 and June 1955 a clinical trial was held in which patients with operable breast carcinoma were subjected to a radical mastectomy and then randomised to either have immediate post-operative radiotherapy (radiated group) or delayed radiotherapy on recurrence (watched group). Data relating to the 1461 patients entered in the trial have been analysed to investigate the late effects of treatment, if any, over a period of 34 years. A logrank comparison of the survival patterns of the radiated and watched groups, considering all deaths, during the first 15 years of follow-up did not show any statistically significant difference (P = 0.37). However, after 15 years there was a significantly increased mortality in the radiated group (P = 0.0025). The relative risk after 15 years for the radiated group relative to the watched group was 1.43 with a 95% confidence interval of 1.13 to 1.81. Taking the series as a whole, this increased mortality was attributable to deaths from cardiovascular disease (excluding cerebrovascular disease). There was no evidence that the increased mortality due to cardiovascular disease was significantly different between patients who had a left- or right-sided tumour. There was also no significant difference between the watched and radiated group from mortality due to breast cancer or other malignancies. The data analysed here relate to patients treated 40 years ago. Neither the type of surgery nor the techniques and quality of radiation are used any longer. Furthermore, an artificial radiation menopause has also fallen into disuse. Data should be prospectively gathered from more recent trials, to see if there is any hazard in the very long term from present day methods of treatment. These should include patients treated by lesser forms of surgery, supervoltage therapy, adjuvant hormone and/or chemotherapy.