Hairy cell leukemia is a mature B-cell non-Hogkin lymphoma characterized by unique clinical, morphological and immunhistochemical features. Patients with hairy cell leukemia usually present with splenomegaly, progressive pancytopenia and a relative indolent clinical course. The diagnosis does not always indicate immediate treatment, as treatment depends on the clinical stage of the leukemia. Asymptomatic disease without progression requires a watchful waiting policy, while other categories usually need treatment. The treatment of choice is purine nucleoside analogues (pentostatin, cladribine) which can achieve complete remission even for decades. Interferon and monoclonal CD20 antibodies can also significantly prolong event-free survival. Unfortunately, only the latter two therapies are easily available in Hungary. Splenectomy, which was suggested as first line treatment before the era of purine nucleoside analogues, is only recommended as a last resort. Although hairy cell leukemia is a well-defined lymphoproliferative disease, sometimes it is difficult to differentiate it from other similar entities such as hairy cell leukema variant, splenic marginal zone lymphoma, small lymphocytic lymphoma etc. Making the correct diagnosis is of utmost importance because of the great difference in treatment modalities. Recently, a somatic mutation was found in all analysed hairy cell leukemia samples, but not in other splenic B-cell lymphomas. This article reviews the significance of this observation and presents the different types of methods for the detection of this mutation.