The rationale for antileukemic therapy in hairy cell leukemia is to reduce the significant risk of infection and other potential serious complications. Corticosteroids have limited value; both corticosteroids and chemotherapy are associated with substantial risks of infection. The mainstay of therapy has been splenectomy. Improvement is seen in 50% to 70% of patients with cytopenias; although the impact of splenectomy on survival has not been clearly demonstrated, prolonged hematologic improvement can occur. Splenectomy presumably alleviates the pancytopenic effect of hypersplenism by removing the preferred site of leukemic cell proliferation. Human interferon represents a major advance in management. Favorable results with natural leukocyte alpha interferon have been confirmed by data with biosynthetic (recombinant) alpha interferon. Importantly, the incidence of infection has been clearly shown to decrease, suggesting improved survival in patients with advanced hairy cell leukemia. Many questions regarding interferon therapy remain unanswered, including optimal dose, optimal duration, and maintenance therapy after maximal response. The mechanism of action is unclear, but possibly interferon modulates as yet unidentified lymphokines or growth factors. In vitro evidence suggests a direct antiproliferative effect of type I interferon on hairy cells. Preliminary data suggest that although toxicity issues, including induction of immunodeficiency and renal insufficiency require further clarification, deoxycoformycin, an adenosine deaminase inhibitor, is also highly effective and holds substantial promise as an important therapeutic modality.