Abstract Introduction Breast conserving surgery (BCS) is a standard procedure for early breast cancer and resection margin state is the most important risk factor of local recurrence. Re-operation is generally conducted in 20∼40% after initial BCS to achieve negative margins, especially in breast cancer with carcinoma in situ components. In this study, we analyzed the long-term follow up results and efficacy of BCS using intraoperative frozen section analysis to access resection margin in ductal carcinoma in situ (DCIS) patients. Methods Between 2004 and 2006, 1016 patients were diagnosed with primary breast cancer and received breast cancer surgery. Among them, BCS was attempted as an initial operation for 523 patients. Superior, inferior, medial and lateral margin of resected specimen were evaluated according to the intraoperative frozen section analysis. If tumor cells existed less than 2mm from resected specimen margin, intraoperative further resection was done and if the further resection was impossible, initial BCS was converted to mastectomy. All medical records and pathologic reports were reviewed retrospectively. Results Of the 523 patients who had to undergo BCS, 13.3% (70/523) were converted to mastectomy during initial BCS. The number of the patients who had either only DCIS or invasive carcinoma c carcinoma in situ component was 372 (71.1%, 372/523) and 17.2% (64/372) were converted to mastectomy. One hundred fifty one (28.9%, 151/523) patients had only invasive carcinoma and 3.97% (6/151) were converted to mastectomy. In this study, we analyzed 94 patients who had to undergo BCS with DCIS. The rate of intraoperative conversion to mastectomy was 13.8% (13/94) and 81 patients had successful BCS with 0∼3 times of intraoperative frozen section analysis. There were no differences between patients who had BCS and final mastectomy in clinicopathologic characteristics such as physical examination of tumor, age of patients, DCIS subtypes, nuclear polymorphism, presence of necrosis, ER, PR, HER2 and Ki67. After permanent biopsy was reported, in 5 patients, resected specimen had tumor cells within less than 2mm from resected margin, not inked margin. They had no reoperations and no recurrences. Mean follow up period was 76.6 months. One locoregional and 3 local recurrences in BCS patients and 1 local recurrence in mastectomy patients were found. There was no difference in disease free survival between two groups (95.1% vs 92.3%, p=0.659). In these DCIS patients, reoperation rate was 0%. Conclusion Intraoperative frozen section analysis during BCS to access resection margin helps to avoid reoperations and increase intraoperative success rate of BCS in DCIS. It also shows oncological safe long term results. Further studies are needed to resolve the problem with cost-effectiveness of intraoperative frozen section analysis. Citation Format: Choi JE, Yeu KJ, Park JY, Kang SH, Lee SJ, Bae YK. The efficacy and long term results of intraoperative frozen section analysis to access resection margin in ductal carcinoma in situ. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-13-06.