Analysis of sentinel lymph node (SLN) by means of One-Step Nucleic Acid Amplification (OSNA) is being used increasingly as a very sensitive and quick method for intraoperative axillary staging in patients with breast cancer. This molecular diagnostic assay detects the expression level of cytokeratin 19 (CK19), a luminal epithelial cell marker broadly expressed in most breast carcinomas and not normally found in lymph nodes. Almost all breast cancers express this cytoskeleton protein, but some breast tumors have been found to lose the expression of CK19. CK19 immunostaining in core biopsies has been recommended in selecting patients eligible for OSNA analysis because SLNs with metastatic involvement by CK19-negative breast cancers may result in a false negative result by OSNA. However, the real frequency of CK19-negative breast cancer has to be elucidated. In this study, we have assessed the frequency and molecular profile of CK19-negative breast carcinomas in three series of cases. The first is a prospective series of 197 breast carcinomas, 111 of which were subjected to SLN evaluation by OSNA. The second is a retrospective series of 41 triple-negative (TN) breast carcinomas, and the third is a retrospective series of 68 breast cancer patients (matched core biopsies and metastatic lymph nodes) that had been evaluated by conventional procedures before the OSNA methodology was adopted in our institution. Our results not only demonstrate that lack of expression of CK19 is infrequent in breast cancers but also that performing CK19 immunohistochemical staining is important on diagnostic core biopsies in taking the decision of using OSNA methodology in the evaluation of sentinel nodes in breast cancer patients.