We studied the effect of postoperative radiotherapy (RT) after breast sector resection for ductal carcinoma in situ (DCIS). The study protocol stipulated radical surgery but microscopically clear margins were not mandatory. We randomised 1,046 operated women to postoperative RT or control between 1987 and 1999. The primary endpoint was ipsilateral local recurrence. Secondary endpoints were contralateral breast cancer, distant metastasis and death. After a median follow-up of 5.2 years (range 0.1-13.8) there were 44 recurrences in the RT group corresponding to a cumulative incidence of 0.07 (95% confidence interval (CI) 0.05-0.10). In the control group there were 117 recurrences giving a cumulative incidence of 0.22 (95% CI 0.18-0.26) giving an overall hazard ratio of 0.33 (95% CI 0.24-0.47, p < 0.0001). Twenty two percent of the patients had microscopically unknown or involved margins. We found no evidence for different effects of RT on the relative risk of invasive or in situ recurrence. Secondary endpoints did not differ. Women undergoing sector resection for DCIS under conditions of population based screening mammography benefit from postoperative RT to the breast. Seven patients needed RT-treatment to prevent one recurrence.