AimsNeoadjuvant therapy is used in many patients with breast cancer before surgery, with the aim of reducing the tumour size, allowing conservative resections. Sentinel node biopsy is a conservative procedure for handling the axilla in breast cancer; however, the use of this technique after neoadjuvant treatment is under discussion. For sentinel node assay, methods based on the detection of cytokeratin 19 (CK19) mRNA, such as one‐step nucleic acid amplification (OSNA), are available. However, if systemic therapy could alter protein expression, then CK19 would not be a good target for analysing these nodes. The aim of this study was to evaluate the immunohistochemical expression of CK19 within different cancer types, and to compare its expression in breast tumours and axillary nodes before and after treatment.Methods and resultsCK19 immunostaining was studied in 162 tumour and node samples before and after treatment. Statistical studies using the McNemar test and chi‐square test were performed. CK19 expression was found in 155 cases. We compared CK19 expression in tumour and node biopsies before and after treatment, and we found a lack of significant CK19 expression changes.ConclusionsOur study has confirmed the preservation of CK19 protein expression in breast cancer cells after neoadjuvant therapy. On the basis of these results, quantification‐based methods such as the OSNA CK19 assay, could be an accurate tool with which to analyse the sentinel nodes, regardless of whether they had been obtained before or after treatment.