This is a review of current opinions on all aspects of diagnosis and treatment of hairy-cell leukemia. First, the authors report a concise but complete description of clinical and hematological characteristics of the disease with particular attention to peripheral blood and bone marrow morphological, cytochemical and cytogenetic features. Still concerning the diagnostic phase, the authors report a brief description of splenic, hepatic and lymph nodal histological pictures and some informations about immunophenotypic diagnosis with monoclonal antibodies. A space is then given to differential diagnosis and prognostic factors which have remarkably changed over the years. The second part of this review concerns current treatments for hairy-cell leukemia and analyses all the aspects of the different therapeutic strategies. Data, for this analysis were obtained from the Italian Cooperative Group's Studies and from other published series. Special attention was given to the impact of splenectomy, alpha-interferon therapy and new drugs such as purine analogues (2-CdA, 2-dCF) on the management of the disease. The last section concerns minimal residual disease, second neoplasia and the most common causes of death. A whole paragraph is dedicated to hairy-cell leukemia variants.