Human T cell leukemia virus (HTLV-II) is an infrequently encountered human T cell leukemia virus first isolated from a patient with atypical hairy cell leukemia. Recently, we identified a second patient infected with HTLV-II who had a similar clinical syndrome of atypical hairy cell leukemia associated with peripheral T cell lymphocytosis. HTLV-II was detected by molecular hybridization studies, and more recently, by electron microscopy, in cell lines derived from the patient. Both patients came from the Los Angeles area and had spent several years in Alaska. As opposed to our two patients, 21 patients with more typical cases of hairy cell leukemia were seronegative for HTLV-II. Two additional cases of unusual T cell malignancy linked to HTLV-II have been described by other investigators and bear limited similarity to our index cases. Further studies are necessary to define the spectrum of malignancies linked to HTLV-II and to identify infected individuals for prospective study.