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The description of hairy cell leukemia as a specific clinical entity was published 50 years ago. The clinical outcome for patients was hampered by ineffective che-motherapy, and splenectomy was the ma-jor therapeutic approach to improve pe-ripheral blood counts. The median survival after diagnosis was 4 years. With the introduction of -interferon in 1984, marked improvements in patient re-sponses were observed. Shortly thereaf-ter, the introduction of the purine nucleo-side analogs transformed this disease into a highly treatable form of leukemia, and patients with the classic form of this rare leukemia now have a near-normal life expectancy. However, other clinical enti-ties mimicking this disease do not re-spond; thus, accurate diagnosis is impor-tant. Immunophenotypic features in classic hairy cell leukemia show that the leukemic cells express CD11c, CD25, CD103, and CD123 and display bright CD20. Despite the high percentage of durable complete remissions with mod-ern therapy, the long-term disease-free survival curves have not reached a pla-teau. Many patients who achieve a com-plete remission by morphologic criteria have minimal residual disease demon-strable by either flow cytometry or immu-nohistochemical staining, and this popu-lation may be at higher risk for earlier relapse. Continued clinical research is essential to optimize therapy for this dis-ease. (Blood. 2010;115:21-28