Between 1969 and 1975, postoperative radiotherapy was performed at the J. Bordet Institute on 787 stage I-III operable breast cancer patients. Crude 10 years survival rates are respectively 61% for stage I, 59% for stage II and 30% for stage III. The results have been analysed according to clinical characteristics of significant prognostic value and according to the technical conditions of treatment. The status of the axillary nodes appears to be the most important prognostic factor. Radiotherapy to the thoracic wall and lymph drainage areas has varied little throughout the whole period with the dose to the internal mammary chain being the only parameter which varied significantly. Loco-regional recurrences occurred only in 5, 2% of patients without distant metastases appearing previously or during the following year (2.4 for stage I patients). Severe sequelae were seen in only a very few cases. Survival appears significantly correlated with an adequate dose of irradiation to mammary node chain.