BACKGROUND Ductal carcinoma in situ (DCIS) of the breast is a disorder that has become more common since it may manifest as microcalcifications that can be detected by screening mammography. Since selected women with invasive cancer can be treated safely with breast conservation therapy it is paradoxical that total mastectomy has remained the standard treatment for DCIS. We did a randomised phase III clinical trial to investigate the role of radiotherapy after complete local excision of DCIS. METHODS Between 1986 and 1996, women with clinically or mammographically detected DCIS measuring less than or equal to 5 cm were treated by complete local excision of the lesion and then randomly assigned to either no further treatment (n=503) or to radiotherapy (n=507; 50 Gy in 5 weeks to the whole breast). The median duration of follow-up was 4.25 years (maximum 12.0 years). All analyses were by intention to treat. RESULTS 500 patients were followed up in the no further treatment group and 502 in the radiotherapy group. In the no further treatment group 83 women had local recurrence (44 recurrences of DCIS, and 40 invasive breast cancer). In the radiotherapy group 53 women had local recurrences (29 recurrences of DCIS, and 24 invasive breast cancer). The 4-year local relapse-free was 84% in the group treated with local excision alone compared with 91% in the women treated by local excision plus radiotherapy (log rank p=0.005; hazard ratio 0.62). Similar reductions in the risk of invasive (40%, p=0.04) and non-invasive (35%, p=0.06) local recurrence were seen. CONCLUSIONS Radiotherapy after local excision for DCIS, as compared with local excision alone, reduced the overall number of both invasive and non-invasive recurrences in the ipsilateral breast at a median follow-up of 4.25 years.