Serum from 60 patients with hairy cell leukemia (HCL) were studied for the presence and the titers of antibodies to Epstein-Barr virus (EBV), cytomegalovirus (CMV) and human T-cell lymphotrophic viruses (HTLV). Eighty-three percent of the patients were seropositive for EBV, with a (reciprocal) geometric mean titer (GMT) of 960. Seventy-eight percent of the patients had antibodies to CMV with a GMT of 435. All 21 patients tested for HTLV I and HTLV III were seronegative; only one patient showed detectable antibodies to HTLV II. The potential role of these infections in the physiopathology of HCL is discussed.