Breast-conserving treatment (BCT) is a standard therapy for early breast cancer. Many reports have described the effectiveness of post-BCT radiation therapy. However, the post-BCT local recurrence rate of only 5% to 10% indicates that radiation therapy may be unnecessary in many cases. To accurately select those patients who do not require post-BCT radiation therapy, we investigated the significance of irradiation in BCT by comparing local recurrence rates in irradiated and nonirradiated patients, grouped according to clinicopathological criteria that we evaluated. The patients were divided into two groups: a previous-criteria group and a present-criteria group. The former group included 85 patients in whom only two factors were considered as the criteria for radiation therapy: margin-positivity and lymphatic metastasis-positivity. The latter group included 318 patients in whom three additional factors were also considered: lymphatic invasion, intraductal extension, and metachronous/synchronous bilateral breast cancer. The use of five clinicopathological factors rather than two as the criteria for irradiation led to an increase in the irradiation ratio from 47.1% to 63.2% and a decrease in local recurrence from 12.9% to 2.2%. Because of the short average follow-up period of this study, further careful, regular follow-up and randomized comparative studies are required. It may also be necessary to include the patient age and margin condition as mandatory criteria for irradiation.