The aim of this study was to analyze the effect of the type of local surgical treatment on survival in young women aged less than 40 years with T1 breast cancer. We analyzed data from 3,512 patients aged ≤40 years old who were diagnosed with T1 breast cancer from the Korean Breast Cancer Registry database between January 1988 and December 2006 and underwent either breast-conserving therapy (BCT) or mastectomy. The overall survival (OS) and breast-cancer-specific survival (BCSS) were compared between BCT and mastectomy. Of the 3,512 patients analyzed, 1,951 (55.6 %) underwent BCT, and 1,561 (44.4 %) underwent mastectomy. The median follow-up period was 111.0 (79.0-131.5) months. Overall, the 10-year OS rates for BCT and mastectomy were 95 and 92.1 %, respectively (p = 00004), and the 10-year BCSS rates for BCT and mastectomy patients were 96.9 and 94.9 %, respectively (p = 0.12). In node-negative patients, no significant difference was observed in either the OS (adjusted hazard ratio [HR] 1.072; 95 % CI, 0.750-1.5332, p = 0.704) or BCSS (adjusted HR 0.988; 95 % CI, 0.620-1.574, p = 0.960) rate between the BCT and mastectomy groups. In node-positive patients, no significant difference was observed in the OS (adjusted HR 1.634; 95 % CI, 0.982-2.272, p = 0.59) and BCSS (adjusted HR 1.410; 95 % CI, 0.755-2.633, p = 0.281) rates between the BCT and mastectomy groups. In this large, population-based analysis of young women with T1 breast cancer, the OS and BCSS were not different between BCT and mastectomy.