Aberrant expression of cytokeratins (CK) is known to occasionally occur in malignant lymphomas. The monoclonal mouse-anti-human CK cocktail CK22 recognizes keratin polypeptides with a wide range of molecular weights and can be applied in diagnostic panels for tumors of unknown origin. Using tissue microarray technology, we tested 1059 lymphoma and acute leukemia cases, covering the most common disease entities, for aberrant CK expression, using CK22. In total, 866 of the arrayed cases were evaluable (80%), and 13 positive cases (1.5%) were found: 1 out of 230 Hodgkin lymphomas (0.4%), 1 plasma cell myeloma, 2 out of 326 diffuse large B-cell lymphomas (0.6%), 5 out of 18 mantle cell lymphomas (26%), 3 out of 70 small cell lymphomas/chronic lymphocytic leukemias (4%) and 1 out of 27 peripheral T-cell lymphomas, not otherwise specified (4%). Immunostaining was finely granular in most cases, and the total amount of positively staining cells exceeded 10% only in the cases of Hodgkin lymphoma and plasmocytoma. All CK22-positive cases, except for one mantle cell lymphoma, expressed the specific simple epithelial CK8 but not the basal/stratified epithelial CK5/6. Aberrant CK expression can be encountered in a small subset of otherwise characteristic B- and T-cell lymphomas, but not in acute leukemias, which should be considered in difficult differential diagnostic settings.