PURPOSE: Since 2001, breast cancer has been the most common type of cancer observed in Korean women. Due to improved screening and treatment, multiple primary cancer (MPC) in association with breast cancer has been more prevalent. However, there is a lack of information regarding MPC in Korea. Therefore, this study was conducted to provide an overview of the clinical characteristics of MPC that occurs in association with breast cancer. METHODS: We retrospectively investigated 149 patients with MPC, including breast cancer patients, who were treated between July 1989 and December 2005 at the Breast clinic in Asan Medical Center. Patients with contra-lateral breast cancer as well as patients that experienced a recurrence of breast cancer were excluded from this study. We studied the clinical characteristics of patients including the mean age, stage of disease, hormone receptor status, cause of death, common cancers that were combined with breast cancer, the time interval between the onset of breast cancer and other cancers, and mean survival. We considered the cancer to be synchronous if a second cancer has been diagnosed at least six months before the diagnosis of an initial primary cancer, however, other forms of cancer were considered to be metachronous. RESULTS: Of 149 patients, 146 patients were female (98.0%) and 3 were male (2.0%). The mean age of the study group was 51.5, which is higher than the mean age of 47.3 that was observed in breast cancer patients at our hospital during the same period (p<0.001). Forty-four of the patients evaluated in this study had synchronous cases of cancer, whereas the remaining 88 patients had metachronous MPC. In metachronous cancers, the average time between diagnosis with the primary cancer and the secondary cancer was 5.2 yr. The most common cancer found to be combined with breast cancer included cancer of the thyroid (45, 29.2%), cervix and uterus (28, 18.2%), stomach (19, 12.3%), colon and rectum (15, 9.7%), ovary (11, 7.1%) and lung (9, 5.8%). CONCLUSION: Thyroid cancer is the cancer most commonly associated with breast cancer, and the mean age of MPC patients was higher than that of patients who only had breast cancer. Due to the improved survival length and treatment, MPC associated with breast cancer is now a greater concern.