Among the 1760 cases of carcinomas of the breast treated between 1981 and 1996, there were 31 cases of bilateral breast cancer. Of these, 7 were synchronous tumors and the remaining 24 patients had metachronous tumors : 41.7% of all patients had presented within 5 years since the first primary tumors were detected and 73.3% of all patients had presented within 10 years. The mean age of the synchronous bilateral cancers was 39.9 (26~55), and the mean age at the diagnosis of the first cancers in the metachronous cancers was 44.75 (31~70) years old. The location of the tumor was the same in 64% and 68% of the synchronous and metachronous cancers, respectively, that had an identified histology. The mean survival of metachronous group was 70.5 months and that of synchronous group was 114.7 months. However, there was no statistical significance between the mean survival times of the two groups (p>0.05). We recommend a careful check-up for the opposite breast at the time of primary cancer treatment, as well as well an organized follow-up program for all patients having undergone treatment for breast cancer. Further investigations are required to determine the incidence of and risk factors for second cancer development in the opposite breast.