Alpha-interferon (alpha-IFN) treatment is highly effective in normalizing the clinical, hematologic, and immunologic parameters of patients with hairy cell leukemia (HCL). Complete remissions (CR), however, are rare, and a few patients do not respond adequately to alpha-IFN. That the poor response to alpha-IFN treatment could be related to a particular immunologic surface marker profile of the HC was investigated in this study. The results showed that most patients who do not respond adequately to alpha-IFN HC have a peculiar immunologic phenotype with a positive response to the Leu1 (CD5) monoclonal antibody, usually absent on HC but characteristically expressed on B-chronic lymphocytic leukemia cells. Of nine HCL patients with this phenotype, only three had partial remissions (PR) and six minor responses (MR) compared with the three CR, 16 PR, and three MR observed in the 22 Leu1 (CD5)-negative patients. The authors postulate that a more extensive immunologic analysis of HCL patients at diagnosis may be predictive of the response to IFN treatment.