In order to analyse the influence of low radiation doses on human rectal adenocarcinomas, gross and microscopical changes after preoperative radiation were compared to controls treated with immediate surgery in a randomised, prospective trial. The X-ray doses given were 31.5 Gy in 3.5 weeks, and the interval between radiation and operation was 2 to 3 weeks. A total of 138 patients having preoperative radiotherapy and 131 controls were analysed. The overall tumour size was reduced after radiation. Complete tumour regression was obtained in six (4.4%) patients. All of these tumours were exophytic and mobile at the initial examination and all were either well or moderately well differentiated. A significant downstaging was found after preoperative radiation. The incidence of positive lymph nodes was 27.5% in the resected specimens in controls and 18.4% after radiation (p less than 0.05). The total number of recurrences was reduced after radiation in stage C2 tumours, but not in the other stages. Preoperative radiation did not influence the histological grade of the tumours. There was no difference between the two randomised groups with respect to 5-year survival or disease-free survival in any histopathological stage.