BACKGROUND Chronic lymphocytic leukemia (CLL) may evolve into large cell lymphoma (Richter's syndrome), prolymphocytic leukemia, acute lymphoblastic leukemia, and myeloma. METHODS A patient with CLL that transformed into a lymphoma of true histiocytic type is described, and the literature on the association of these two disorders is reviewed. RESULTS Lymphomas of true histiocytic type developing as an aggressive terminal phase of CLL previously have been reported in nine patients. Fever and rapidly increasing lymphadenopathy and splenomegaly were the most common signs and symptoms. As with de novo lymphoma of true histiocytic type, extranodal involvement of the soft tissue, gastrointestinal tract, kidneys, bone marrow, liver, and lungs was documented among the 10 patients with lymphoma of true histiocytic type transformed from CLL. The median interval between diagnosis of CLL and the evolution to lymphoma of true histiocytic type was 25.5 months. Patients with lymphomas of true histiocytic type after CLL were treated with fludarabine and various other combination chemotherapy regimens with only short-lived responses. The median time to death after transformation was only 33 days (range, 10 days to 5 months). CONCLUSIONS Lymphomas of true histiocytic type appear to represent an additional, though uncommon, form of transformation in CLL. Although their presentation is reminiscent of other intermediate to high grade lymphomas, they can be distinguished based on their morphologic and immunophenotypic features. In the patients described in this study to date, transformation of CLL to lymphomas of true histiocytic type is a poor prognostic sign, with survival generally of only days to weeks.