PURPOSE: To compare the treatment outcomes and to analyze prognostic factors between the use of a breast-conserving treatment (BCT) and a mastectomy for early stage breast cancer. MATERIALS AND METHODS: We retrospectively reviewed 1,200 patients with pathological stage T1-2N0 breast cancer who received surgery between September 1994 and December 2002 at Samsung Medical Center. We compared the patient characteristics and treatment outcomes between the two treatment groups. RESULTS: Among the 1,174 eligible patients, 601 (51.2%) patients received a BCT and the remaining 573 (48.8%) patients received a mastectomy. The mastectomy group of patients had significantly more cases with a larger tumor size, multicentricity, extensive intraductal component, and estrogen- and progesterone-receptor negativity. The ten-year overall survival rates (OS) of the BCT and mastectomy groups were 91.96% and 91.01%, respectively (p=0.1274). The ten-year disease-free survival rates (DFS) were 80.48% for the BCT group of patients and 84.95% for the mastectomy group of patients, respectively (p=0.8795). CONCLUSION: Our study shows some differences in patient characteristics between the two treatment groups. However, these differences did not result in significant survival differences.