The paper shows indications and technology for determination of resection margins during organ-sparing surgery for breast cancer. Objective: to conduct intraoperative emergency cytological and histological examination of resection margins during organ-sparing surgery for breast cancer. Subjects and methods. The P.A. Herzen Moscow Oncology Research Institute performed emergency cytological examination of 754 patients in 2009 to 2013. A total of 1661 breast resection margins during radical surgery were investigated. Tumor cells were found at the margin of resection in 2.5% of the patients. False-negative cytological rates were 1.2%. The significance of the investigation was 98.8%. Results. The investigation has provided evidence that cytological examination of resection margins is a full-fledged method, as well as histological examination. The results of intraoperative emergency cytological and histological examinations were compared with those of planned histological examination of resection margins. After defining a planned histological response, a reoperation was performed in 3 cases: removal of the remaining part of the breast in 2 cases and subcutaneous mastectomy with one-stage repair in 1. The removed breast part was found to contain sets of cancer cells in 2 cases; no tumor was detected in 1 case. One patient has refused a second reoperation and is being followed up. Conclusion. To achieve pathologically negative resection margins should be a main goal of organ-sparing surgery for breast cancer. Cytological examination of resection margins is a full-fledged method, as well as histological examination.