As radiotherapy following surgery for early breast cancer carries morbidity, financial cost, and potential mortality we have sought to select characteristics that will allow these risks to be minimised. The causes of increased mortality shown in the overview of mature trials of radiotherapy have been studied in a single large randomised trial. This has identified patients with large, poorly differentiated tumours to be most likely to benefit from the reduction in incidence of local recurrence achieved by radiotherapy while remaining at the lowest risk from suffering the increased mortality associated with the treatment. Much of the morbidity of radiotherapy is due to routine irradiation of lymphatic pathways. Three hundred and seven patients treated by breast-conserving surgery and breast radiotherapy have been evaluated. None received irradiation to their axilla or internal mammary lymphatics. Relapse in locoregional lymphatics (axilla or supraclavicular fossa) occurred in 15 patients. Only 4 had persisting symptoms from this relapse following further treatment.