HCL is a well-recognized entity among the lymphoproliferative disorders. With better appreciation of the wide variability in its clinical and hematologic manifestations, some authors have proposed several subtypes of HCL such as leukopenic and nonleukopenic subtypes and subtypes with and without massive splenomegaly. As opposed to such a clinical and hematologic variability, the pathology of HCL in the spleen and bone marrow is consistent and highly characteristic. Since the spleen becomes available for pathologic examination only after therapeutic splenectomy, the bone marrow pathology often plays the most important role in the differential diagnosis of HCL. It is characterized by focal or diffuse mononuclear cell infiltration with a wide spacing between individual nuclei in most patients and by a severely hypocellular marrow with individual hairy cells infiltrating between the marrow fat cells in the remaining minority of patients. The bone marrow biopsy also serves as one of the criteria in selecting the therapeutic modality as well as monitoring the therapeutic effect. Further, new insights into the pathogenesis of HCL are emerging from recent studies of the bone marrow microenvironment.