We describe the clinical and laboratory features of 17 adult patients with a variant form of hairy cell leukemia (HCL-V) studied over the last 7 years. The main findings were: splenomegaly, moderate anemia, thrombocytopenia, and a raised white blood cell count (median 116 x 10(9)/L; range 15 to 482). The circulating lymphoid cells had abundant villous cytoplasm and a round, occasionally bilobed nucleus, with a prominent nucleolus. Monocytopenia, a feature of typical HCL, was not seen; neither was tartrate-resistant acid phosphatase demonstrated in eight cases tested. HCL-V cells had a mature B-cell phenotype: CD19+, CD20+, CD22+, FMC7+, CD11c+, CD10-, CD5-, with light chain isotope restriction in 15 cases. In contrast to typical hairy cells, HCL-V cells were negative with the monoclonal antibodies anti-HC2 and anti- TAC (CD25). Immunoglobulin (Ig) was not detected in two cases and IgG was expressed in the cell membrane of 73% of cases. Bone marrow histology was different from HCL, showing interstitial infiltration by cells clumped together and a moderate amount of reticulin, but the spleen showed the typical red pulp expansion of HCL. HCL-V patients did not respond to splenectomy (5 of 7) or alpha-interferon (7 of 7); 2 of 3 patients had a partial response to 2′deoxycoformycin. The clinical course was benign with 15 patients alive with a median survival greater than 4 years. We confirm that HCL-V is a distinct clinico-pathologic entity with intermediate features between HCL and B-prolymphocytic leukemia.