Intraoperative frozen section of the sentinel lymph nodes in breast cancerIntroduction: The sentinel lymph node (SN) biopsy has become the standard technique for staging the axillary involvement in breast cancer. The intraoperative diagnosis of lymph node metastatic involvement can immediately decide the need for axillary dissection. The main objective of this study was to determine the accuracy of intraoperative frozen section in the investigation of metastases in the SN, in patients with breast cancer, between 1999 and 2011. Materials and Methods: A retrospective study of patients operated for invasive breast cancer, in which the SN was studied with intraoperative frozen section. Results: 503 patients biopsy was performed with intraoperative frozen section. One hundred fifty-four (30.6%) patients had metastases in the SN, of these 103 (66.9%) were detected on frozen section. No significant difference in the ability to detect metastases for tumors up to 2 cm and tumors more than 2 cm was observed (p = 0.86). The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of frozen section for detection of metastases in the SN was 66.9%, 99.7%, 99% and 87%, respectively. Conclusions: The accuracy of intraoperative biopsy for detection of metastases in the SN in our center is comparable to that described in the literature, being effective only for the investigation of macrometastases (MA). Given the low frequency of metastases in the SN in tumors up to 2 cm, associated with a relatively high number of micro (MI) and submicrometastasis (SM) and false negatives, it seems that in these tumors intraoperative biopsy is unnecessary.