PURPOSE: A medullary carcinoma of the breast(MC) is a well-circumscribes tumor composed of poorly differentiated cells growing in a syncytium with an accompanying stroma. However, the prognosis of a MC is considered as more favorable than that of an infiltrating ductal carcinoma (IDC). In the present study, we characterized MC in terms of prognosis by comparing an MC group with an IDC control group. We described the distribution of other clinicopathological characteristics, as well as the prevalence and the prognostic importance of generally well known risk factors, for breast cancer and compared the result. MATERIALS AND METHODS: Clinical data from hospital records and pathological materials were available from 60 patients with tumors that had been initially diagnosed from Jan. 1981 to Dec. 1999 at the Department of Surgery in Seoul National University Hospital as having a MC. We analyzed the survival and the prognostic factors of those patients and compared the results with those for an IDC control groep. RESULTS: The 60 cases of MC showed more risk factors, such as young age, high nuclear grade, poor histologic grade, negative hormone receptors, p53 overexpression, c-erb-B2 expression, and high proliferative index(ki 67), than the IDC cases did. However, the prognosis of MC was better than that of IDC. Most of the risk factors were of highly significant prognostic importance in the IDC control group. In the MC group, only lymph-node status and young age were significantly important for disease-free survival. CONCLUSION: We found MC to be biologically unique, and patients with MC have a better prognosis than those with IDC. We propose that MC patients with axillary lymph-node metastasis and young age should be considered as a high-risk group for recurrence.