A systematic review of radiation therapy trials in several tumour types was performed by The Swedish Council of Technology Assessment in Health Care (SBU). The procedures for evaluation of the scientific literature are described separately (Acta Oncol 2003; 42: 357-365). This synthesis of the literature on radiation therapy for breast cancer is based on data from 29 randomized trials, 6 meta-analyses and 5 retrospective studies. In total, 40 scientific articles are included, involving 41 204 patients. The results were compared with those of a similar overview from 1996 including 285 982 patients. The conclusions reached can be summarized as follows: There is strong evidence for a substantial reduction in locoregional recurrence rate following postmastectomy radiation therapy to the chest wall and the regional nodal areas. There is strong evidence that postmastectomy radiation therapy increases the disease-free survival rate. There are conflicting data regarding the impact of postmastectomy radiotherapy upon overall survival. There is strong evidence that breast cancer specific survival is improved by postmastectomy radiotherapy. There is strong evidence for a decrease in non-breast cancer specific survival after postmastectomy radiotherapy. There is some evidence that overall survival is increased by optimal postmastectomy radiation therapy. There is strong evidence that postmastectomy radiotherapy in addition to surgery and systemic therapy in mainly node-positive patients decreases local recurrence rate and improves survival. There is moderate evidence that the decrease in non-breast cancer specific survival is attributed to cardiovascular disease in irradiated patients. There are conflicting data whether breast conservation surgery plus radiotherapy is comparable to modified radical mastectomy alone in terms of local recurrence rate. There is strong evidence that breast conservation surgery plus radiotherapy is comparable to modified radical mastectomy alone in terms of disease-free survival and overall survival. There is strong evidence that postoperative radiotherapy to the breast following breast conservation surgery results in a statistically and clinically significant reduction of ipsilateral breast recurrences followed by diminished need for salvage mastectomies. There is strong evidence that the omission of postoperative radiotherapy to the breast following breast conservation surgery has no impact on overall survival. In one meta-analysis including three randomized studies a survival advantage is demonstrated by Bayesian statistics. There is strong evidence that the addition of a radiation boost after conventional radiotherapy to the tumour bed after breast conservation surgery significantly decreases the risk of ipsilateral breast recurrences but has no impact on overall survival after short follow-up. There is strong evidence for the use of postoperative radiotherapy to the breast following breast conservation surgery for DCIS (ductal breast cancer in situ). Radiotherapy leads to a clinically and statistically significant reduction of both non-invasive and invasive ipsilateral breast recurrences. There is insufficient evidence to define the optimal integration of systemic adjuvant therapy and postoperative radiotherapy. There are limited data on radiotherapy-related morbidity in breast cancer. No conclusions can be drawn.