In the presence of clinically negative axillary lymph nodes, urgent intraoperative morphological examination of sentinel lymph nodes (SLNs) is now widely used in early breast cancer (BC). The study of SLNs in early BC for metastases is a predictor of non-SLN involvement and determines the volume of lymph node dissection. Objective — to assess the diagnostic value of an urgent SLN cytological study in early BC. Subject and methods. In August 2016 to September 2018, the P.A. Herzen Moscow Oncology Research Institute conducted a study of 837 SLNs from 609 patients with early BC. SLNs were identified using the radioisotope colloid Technefite-99mTc and single-photon emission computed tomography (SPECT) and SPECT/CT scintigraphy. A portable NEO2000I gamma detector was intraoperatively used to detect SLNs. The latter was incised in the long axis and then the surface of the incision was scraped off. A Leucodif staining kit (Czech Republic) was applied for urgent staining of cytological specimens. The studied lymph node was sent for a planned histological examination. The results of the urgent cytological study were compared with those of the planned histological conclusion. Results. Cytology revealed metastases in 88 (10.5%) SLNs in 68 patients, i.e. in every 9 patients, including in 5 cases in the presence of micrometastases. The planned histological examination showed metastases in 139 (16.6%) SLNs in 116 patients, i.e. in almost every 5 patients; of them 22 (16%) cases had micrometastases. Thus, metastases were not cytologically found in 51 SLNs from 48 patients. Conclusion. The sensitivity, specificity, accuracy, and efficiency of SLN cytology were 94.6, 99.7, 99.6 and 91.4%, respectively. Urgent cytological examination of SLNs could identify macrometastases (71%) and micrometastases (23%). The effectiveness of cytological studies of SLNs is explained by the presence of micrometastases in 16% of cases, and in other cases, by the small macrometastases that did not lie in the plane of the incision (20% of all macrometastases). Lymph node metastases almost always begin to form in the subcapsular cortical layer and not all metastases fall into the plane of the incision from which a scrape is taken. Cytological detection of micrometastases is largely an accident (23% of micrometastases were cytologically detected). True cytological hypodiagnosis (single tumor cells were missed by a cytologist) was 0.5%.