Hairy-cell leukemia (HCL) is a lymphoproliferative B-cell malignancy--it represents about 2% of all adult leukemias. HCL is associated with pancytopenia and splenomegaly. In the late 1980s, introduction of new purine analogs such as 2-deoxycoformycin (pentostatin, DCF) and 2-chlorodeoxy-adenosine (2-CdA) significantly improved the prognosis of HCL patients. 33-89% patients can achieve a complete remission (CR) following DCF treatment and 85% CR after 2-CdA therapy. There is no cross-resistance between pentostatin and 2-CdA. Residual hairy cells are present in bone marrow of almost all patients after purine analogs therapy, detected by immunohistochemical methods. It is called minimal residual disease (MRD). The spleen may be the source of MRD after purine analogs therapy. Thus splenectomy could be a profitable approach after chemotherapy. Hairy-cell leukemia relapse appears in 47.8% of cases in 30 months after pentostatin treatment and in 23% of cases in 3 years after 2-CdA therapy. There is no perfect treatment of HCL relapse. Thanks to new purine analogs hairy-cell leukemia may be considered a potentially curable disease.