Gene expression analyses with cDNA microarray technology identified distinct groups of breast cancers. Tumors with no ER expression could be divided into three subgroups: "basal-like" subtype, HER2-positive subtype, and "normal breast-like". "Basal-like" subtype was characterized by high expression of keratins 5 and 17, laminin and fatty acid binding protein 7. In the present study, we analyzed the usefulness of immunohistochemistry for separation of the distinct subtypes of the breast ductal carcinomas and provided further characterization of "basal-like subtype". A consecutive series of 195 primary operable invasive breast carcinomas was immunostained for HER2, ER, PGR, CK5/6 and CK17. CK5/6 or CK17 were expressed in 72 cases (36.9%), and 41 cases (21%) presented expression of CK5/6 or CK17 without ER/PGR or HER2. ER/PGR was present in 109 cases (55.9%), but in this group there were 8 cases with HER2 overexpression and 17 cases with basal-cytokeratin positivity. Similarly, in 17 out of 72 "basal-like" tumors there was ER/PGR positivity, and also in 17 of them there was HER2 overexpression. Three of these cases belonged to all three groups, representing expression of all markers. Tumor grade differed significantly (p < 0.001) between luminal and basal cytokeratin- or HER2-positive tumors. Differences for tumor size and lymph node status were not statistically significant. Our study showed that immunohistochemistry is useful for dividing breast cancers into separate subgroups, but further analyses for better characterization of cases presenting two or three markers should be performed.