Diagnosis of minimal infiltration by hairy cell leukemia (HCL) cells in bone marrow trephines is challenging as its differentiation from interstitially distributed infiltrates of reactive B cells and small B-cell lymphomas other than HCL is difficult or impossible. The diagnostic utility of DBA.44, tartrate-resistant acid phosphatase (TRAP) and of the recently propagated Annexin A1 is limited in this diagnostic setting since DBA.44 does not always label all HCL cells, TRAP is frequently weakly expressed whereas the numerous Annexin A1-positive hematopoietic cells interfere with the immunohistochemical recognition of sparse HCL cells. Therefore, we searched for antibodies suitable for the identification of minimal HCL infiltrates in formalin-fixed bone marrow trephines. Among the antibodies tested those directed against the T-cell associated transcription factor T-bet showed the required specificity: (1) T-bet is highly expressed in all 85 cases of HCL including 31 cases showing only minimal infiltrates, and (2) T-bet is exclusively expressed only on a subset of reactive T cells which are rare in bone marrow. Although T-bet is also weakly expressed in a proportion of the tumor cells of some small B-cell lymphomas (n=32/69) other than HCL, a reliable distinction is nevertheless easily possible because (1) T-bet is strongly expressed by all hairy cells of all HCL cases and (2) T-bet is not coexpressed in HCL with markers typical for other small B-cell lymphomas/leukemias (ie, CD23, CD5, CD10, and/or BCL6). Therefore, the immunohistochemical detection of T-bet in infiltrated bone marrow trephines represents an important adjunct for the diagnosis of HCL.