This report describes 150 patients with clinical stage I and II carcinoma of the breast treated at four institutions--Yale University School of Medicine, Harvard Medical School-Joint Center for Radiation Therapy, Hahnemann Medical College, Jefferson Medical College--with radiotherapy only following excisional biopsy. Closely similar treatment policies were followed at all four centers, 4500-5000 rads minimum tumor dose being delivered to the entire breast and axillary, supraclavicular and internal mammary nodes. Forty-six of 49 stage I patients treated are alive without disease, the actuarial relapse-free survival being 91% at 5 years. Of the 101 stage II patients, 75 are alive without disease with a relapse-free actuarial survival of 60% at 5 years. Local failure has occurred in 10 patients (9 stage II and 1 stage I, 6.6%) 5 of whom are disease-free following mastectomy. The results obtained in this study are comparable to those of conventional surgery. It is our conclusion that mastectomy is not a necessary part of the treatment of small breast cancers, that radiation without mastectomy is an acceptable alternative with far superior cosmetic and functional results. Adjuvant chemotherapy should be considered particularly in stage II patients in view of their 40% relapse rate.