BACKGROUND Cytokeratin s (CK) are used for the fingerprinting of carcinomas in general. In breast tissue, the luminal epithelial cells express CK 8/18, CK 7 and CK 19, while basal/myoepithelial cells express CK 5/6, CK 14 and CK 17. METHODS Immunohistochemical staining for cytokeratin 5/6 was applied on cell block sections of 23 cases of benign and 25 cases of malignant breast lesions using avidin biotin peroxidase technique. The distribution and intensity of staining was recorded and graded semiquantitatively. RESULTS All benign lesions showed positive immunoreaction, with the staining index varying from 6-9, except lactating adenoma. The malignant lesions comprised three cases of ductal carcinoma in situ (DCIS) and 22 cases of infiltrating ductal carcinoma, not otherwise specified, IDC (NOS). None of the DCIS cases showed a positive immunoreaction. Among the IDC (NOS) lesions, six cases of grade III breast carcinoma exhibited a positive immunohistochemical reaction, the staining index of which varied from 2-6. The staining reaction in the malignant lesions was only cytoplasmic and the intensity was significantly less than that of benign lesions. CONCLUSIONS CK 5/6 expression breast carcinoma implies a 'basal like' molecular phenotype and is associated with poor prognosis. This antibody is also used as a component of panels to differentiate benign and malignant breast lesions.