Radiotherapy (RT) is not always necessary for the prevention of ipsilateral breast recurrence in cases where cancer is not detected in the remaining breast tissue after breast conserving surgery. In addition, under these circumstances, the rate of a second primary cancer of the remaining breast is theoretically equal to the rate of contralateral breast cancer. In performing breast conserving treatment (BCT) at our institution we do not treat with RT if a strict serial pathological examination of the specimen (every 5 mm) reveals that the case has been safely resected (negative surgical margins). From 1986 to 1998, 827 patients (157 were ductal carcinoma in situ, and 670 were invasive) underwent BCT without RT at the Cancer Institute Hospital. Ipsilateral breast cancer was observed in 46 cases or 5.6% (0.85% annually) during a median observation period of 67 months. Of these 46 cases, 19 (2.3%) were diagnosed as a recurrence and 27 cases (3.3%) were second primary cancers. This recurrence rate is equivalent to the rate observed in 406 cases of BCT (1.7%) that were treated with RT. Most of these cases had shown positive surgical margins. Furthermore, the rate of occurrence of second cancers is not significantly different from the rate of occurrence of contralateral breast cancers. These results suggest that, by selecting irradiation cases based on careful pathological examinations, BCT can be safely performed.