In the paper recent advances in etiology and pathogenesis of hairy cell leukaemia (HCL) are reviewed. Role of human T-cell leukaemia virus II (HTLV-II) and Epstein-Barr virus as well as benzene and other environmental factors in the etiology of this leukaemia is presented. The new achievements in diagnosis including electron microscopic, cytogenetic and immunologic studies are discussed. A rare variants of HCL are described. The special attention is paid to the modern approach in treatment including alfa-interferon, 2'-deoxycoformycin and 2-chlorodeoxyadenosine.