Hairy cell leukemia (HCL) is a chronic B-cell lymphoproliferative disorder characterized by pancytopenia and variable infiltration of the reticuloendothelial system with hairy lymphocytes. The lymphocytes present characteristic citoplasmatic projections and are positive for tartrate-resistant acid phosphatase (TRAP). It is more common in men than women and has a median age of diagnosis of 52 yr. It is typically seen in middle-aged men, characterized by pancytopenia, splenomegaly, immunologic abnormalities, and morphologically typical neoplastic mononuclear cells in the blood, bone marrow, liver, spleen, and other tissues. In addition to pyogenic infections, patients are susceptible to unusual organisms including atypical mycobacterium, legionella, and fungi. Diagnosis for HCL is confirmed by demonstration of hairy cells in biopsy specimens from the bone marrow or spleen or in peripheral blood. Splenectomy is the standard initial treatment of choice. Recently, the nucleosides pentostatin (2'-deoxycoformycin) (DCF) and 2'-chlorodeoxyadenosine (2-CdA) have been shown to produce greater numbers of durable complete remissions with curative potential in patients with HCL.