CD11c (LEU-M5) expression characterizes a B-cell chronic lymphoproliferative disorder with features of both chronic lymphocytic leukemia and hairy cell leukemia. Chronic lymphocytic leukemia (CLL) and hairy cell leukemia (HCL) are two common chronic lymphoproliferative disorders, each having characteristic clinical, morphologic, and immunologic features. Phenotypically, CD5 reactivity in CLL and CD11c (Leu-M5) reactivity in HCL have characterized these two leukemias among B-cell disorders. In this study, we report 14 cases of a novel chronic lymphoproliferative disorder characterized by lymphocytosis and CD11c expression, but morphologically similar to CLL. The patients' ages ranged from 46 to 81 years (median 62). Eleven had palpable splenomegaly, five with markedly enlarged spleens; only one patient had generalized lymphadenopathy. The white blood cell count ranged from 5.2 to 131.0 x 10(9)/L (median 20.8). The morphologic diagnosis in all cases was CLL, with the cells usually having abundant cytoplasm. No morphologic features, of hairy cells were evident; tartrate-resistant acid phosphatase cytochemistry was negative in all cases. Bone marrow biopsies were available in 8 of 14. Four showed focal nodular infiltrates and two had diffuse infiltrates similar to CLL; two showed only minimal interstitial involvement. All cases expressed multiple B-cell markers, and 12 of 14 had monoclonal surface immunoglobulin. The leukemic cells of all cases strongly expressed CD11c, while CD5 was expressed in 7 of 14; only 1 of the 14 cases expressed the lymph node homing receptor, Leu-8. This unique group of leukemias appears to represent the malignant transformation of lymphocytes arising from a stage of lymphocyte differentiation between that found in typical cases of CLL and that of HCL. CD11c is known to have an important function in cellular adhesion and may be important in determining the pattern of lymphocyte tissue distribution found in this group of patients. 