Prospective, randomized clinical trials have demonstrated that the alternatives of mastectomy or conservative surgery plus radiation therapy provide equivalent survival for patients with invasive breast cancer. The identification of a subset of women who could undergo conservative surgery without radiotherapy would avoid the costs, inconvenience, and complications of radiotherapy and is an important research goal. Four randomized trials comparing conservative surgery alone with conservative surgery plus radiotherapy have demonstrated an average reduction in the risk of disease recurrence in the breast of 84% with the use of radiotherapy. No significant differences in survival have been observed, although the available studies lack sufficient numbers of patients to demonstrate a potential small, but clinically important, survival advantage for patients treated with radiotherapy. Subset analysis in the randomized trials and prospective studies of highly selected patients have failed to consistently identify a group of patients who do not benefit from radiation therapy. Any recurrence of breast cancer is psychologically devastating, and fewer than one half of the patients who have had disease recurrence after conservative surgery alone have undergone further breast-conserving treatment. At present, a group of patients who do not require radiotherapy has not been reproducibly identified, and radiotherapy should remain a part of breast-conserving therapy for invasive carcinoma.