Over an 18-year period a distinctive large cell lymphoreticular neoplasm (Richter's transformation) developed in 9 patients with chronic lymphocytic leukemia. Clinical findings at the onset of Richter's transformation were remarkably uniform and consisted of the abrupt onset of fever, marked asymmetric lymphadenopathy with the formation of masses, splenomegaly, and hepatomegaly. All patients underwent rapid clinical deterioration followed by death within six and a half months. Earliest infiltrates of large lymphoreticular cells were identified in the lymph nodes in 3 of 4 patients and the bone marrow in 3 of 9 patients, while no patient had peripheral blood involvement. Autopsy examinations revealed extensive infiltrates of large lymphoreticular cells, predominantly in bone marrow, lymph nodes, liver, spleen, but also in kidney, lung, and gastrointestinal tract. In each case, these large lymphoblast-like and pleomorphic lymphoreticular cells were admixed with mature-appearing lymphocytes and intermediate forms (prolymphocytes). Electron microscopic and immunoperoxidase studies provided additional evidence that this highly aggressive lymphoreticular neoplasm represents a transformation or dedifferentiation of chronic lymphocytic leukemia.