Recent immunophenotypic studies of hairy cell leukemia (HCL) have suggested specific patterns of immunoreactivity that may aid in diagnosis. We studied peripheral blood (PB) from 161 cases of HCL using two-color direct immunofluorescence flow cytometry and an extended panel of antibody combinations. Circulating hairy cells were identified by immunophenotypic features in 92% of the cases and could be detected even when representing < or = 1% of circulating lymphocytes. The 133 cases with > or = 2% detectable hairy cells were analyzed in detail. HCL showed a uniform and unique B-cell phenotype, with each of the following features identified in 99% to 100% of cases: (1) positive staining for B-ly7, coexpressed with CD19; (2) very intense, uniform expression of CD11c, with CD19; (3) moderately intense staining for CD25, with CD19; (4) very intense staining for CD22; (5) moderate to very intense staining for CD20; and (6) moderately intense monoclonal surface Ig. Phenotypic variability existed in expression of CD10 (26%) and CD5 (4%). Based on these features, HCL was easily distinguished from 50 cases of chronic lymphocytic leukemia (CLL). Although CLL exhibited frequent expression of CD11c (74%) and CD25 (68%), the intensity of staining was significantly less than HCL. Furthermore, CLL was uniformly positive for CD5 and showed weak staining for CD20, CD22, and surface Ig. B-ly7 proved to be the most specific marker, reacting with 100% of HCL cases, but absent in all cases of CLL. We conclude that two-color flow cytometry with specific antibody combinations is an efficacious method for characterization and sensitive detection of hairy cells in PB. Application of the phenotypic criteria described should help to increase accuracy in diagnosis of HCL.