Although the figures for long-term survival are small, they show a definite improvement on those for alternative forms of radiotherapy. Some selection of patients is essential; we should appreciate our limitations and c o n h e treatment to cases in which there is reasonable prospect of producing regression of the tumour mass with relief of symptoms. For patients with advanced disease and infection, colostomy alone is much less trying for the patient and results may be achieved more quickly for the remainder of the patient's life. The question of radiotherapy here needs careful consideration, for colostomy may reduce available ports of entry and so make adequate radiotherapy difficult. To attempt the hopeless is worse than useless, and the only indication for radiotherapy in very advanced disease is to try and relieve pain due to invasion of, or pressure on, the pelvic nerves.3. Treatment should be considered whenever the surgeon and pathologist report that the disease has extended beyond the rectal wall. Untreated, 50 per cent of these patients will develop recurrences and metastases from which they will die. Radiotherapy is not advised as a routine pre-operative measure. Profound changes occur in the pelvic tissues following this heavy irradiation and the risks of necrosis are high. 4. We do not consider that the improvement in the effects of these radiations is due to the shorter wave-length of the million-volt X rays. It is not a difference in biological effect, but is due rather to the increased depth-dose and to the greater accuracy, efficiency, and ease of application.