OBJECTIVE to assess the value of radiation therapy in reducing local-regional relapses following mastectomy or breast conserving surgery. METHODS Between 1983 and 1986, 658 women underwent mastectomy and axillary dissection and in 106 cases breast conserving surgery and axillary dissection were done for invasive operable breast cancer. Patients with the same pTpN status were assigned to subgroups according to whether they received radiation therapy or not. RESULTS 1. After mastectomy: pT1pN(zero) status is not an indication for irradiation. 20 mm was defined as a cut off diameter in locoregional recurrence: pT 1/c versus pT2 p = 0.0590. In particular, patients with pT2 tumors, located in the medial-central subregions, had better locoregional results after irradiation (p = 0.0328). The condition of axillary lymph nodes was a significant predictor of locoregional recurrence (p = 0.0000) but between pN(zero) and pN 1/a there was no essential difference (p = 0.9594). 2. After breast conserving surgery: Patients receiving radiation therapy had better results than unirradiated ones (p = 0.0002). The rate of loco-regional relapses was 13.8% and 52.4% respectively. Radiation therapy was able to decrease a tumor bed relapses and even then the diameter of tumor vas < or = 10 mm (p = 0.0218). Local recurrence was more likely in the presence of extensive intraductal components, as opposed to no extensive components (p = 0.002). Overall survival after mastectomy or breast conserving surgery, when patients with the same pTpN status vere compared, was similar (p = 0.6293). CONCLUSIONS after mastectomy pT1N(zero) status is not an indication for irradiation. The critical tumor diameter was 20 mm in the locoregional tumor control. Breast irradiation after conservative surgery resulted in a decrease in local recurrence and even then minimal breast cancer.