In a prospective randomized clinical trial conducted by the European Organization for Research and Treatment of Cancer (EORTC), mastectomy was compared with breast-conserving therapy in 903 stage I and stage II breast cancer patients entering the study between 1980 and 1986. The main participating centers were: Guy's Hospital, London; The Netherlands Cancer Institute, Amsterdam; University Hospital, Leuven; Radiotherapy Institute, Rotterdam; Breast Unit, Tijgerberg, S.A. The data were collected in the EORTC Data Center, Brussels. Treatment in the study arm consisted of lumpectomy, axillary clearance, and radiotherapy to the breast (50 Gy external irradiation in 5 weeks followed by boost with iridium implant of 25 Gy). Important in this study is the large number of TNM stage II patients (755). Most patients were stage II because of the size of the tumor (2-5 cm). The patient and tumor characteristics in the study and control groups were well balanced. So far the survival curves and local recurrence rates are not statistically different for the two study arms. Tumor size was found in univariate analysis to be a significant risk factor for local recurrence in the breast-conserving therapy group but not in the mastectomy group. Results of salvage treatment for local recurrence were not better for the breast-conserving therapy group compared with the mastectomy group. Measurements of quality of life and cosmesis show a clear benefit for the breast-conserving therapy group.