BACKGROUND It is established that immunophenotyping constitutes a useful method in the diagnosis of hairy cell leukaemia. However, no single marker is specific for hairy cell leukaemia. Two-color-flow cytometry can aid in the diagnosis by showing coexpression of CD11c, HC2 or CD25 with pan B cell markers. Recently, Matutes E. et al. [17] proposed a scoring system of immunophenotypic markers, which could be used to evaluate the diagnosis of hairy cell leukaemia. OBJECTIVE The aim of the study was to: a) confirm previous observations of immunophenotypic characteristics of hairy cell leukaemia; b) identify antibody combinations of two-color immunofluorescence staining that are most useful in the diagnosis of hairy cell leukaemia; c) examine the value of a scoring system of immunophenotypic markers in the diagnosis of hairy cell leukaemia. METHODS We analyzed peripheral blood of 46 patients with hairy cell leukaemia using indirect immunofluorescence flow cytometry (EPICS-Coulter) with an extended panel of monoclonal antibodies: CD19, CD2O, CD22, CD24, CD10, HLA-DR, CD11c, CD25, HC2, Slg, kappa and lambda light chains. The diagnosis of hairy cell leukaemia in all patients was made using conventional criteria based on cell morphology, TRAP cytochemistry and bone marrow histology. One fourth of patients were also analyzed using two-color flow cytometry with three antibody combinations as follows: CD19 + CD11c, CD19 + CD25 and CD19 + HC2. RESULTS Our results showed that hairy cells of our patients had a uniform and unique immunophenotype with expression of the following markers: CD19, CD22, CD2O, CD11c and HLA-DR in 100% of patients, CD24 in 93%, CD25 in 88%, Slg in 82%, HC2 in 67%, CD1O in 50%, kappa light chains in 38% and lambda light chains in 35% of patients (Table 1). The level of detectable circulating hairy cells varied widely, from 2% to 93% of total lymphocytes, and 12 patients (26%) with less than 5% of detectable hairy cells were excluded from analysis. Two-color cytometry showed that antibody combination CD19 + CD11c was coexpressed in 100% of patients, CD19 + CD25 in 78% of patients and CD19 + HC2 in 57% of patients (Table 2). Only patients with 5% or more double colored hairy cells for one antibody combination, were included in the analysis. On the basis of our results of in immunophenotyping of hairy cell leukaemia patients and results of other authors (17), we made our scoring system which considers the reactivity with the following markers: CD19, CD11c, CD25 and HC2. Each marker gives 1 point if positive and 0 point if negative. Score 4 had 83% of patients, score 3 had 14% of patients, score 2 had 3% of patients and no patient had score 1 or 0 (Table 3). CONCLUSIONS Our results demonstrated that immunophenotyping with a selective panel of monoclonal antibodies could significantly increase the accuracy in diagnosis of hairy cell leukaemia. Two-color flow cytometry with antibody combination CD19 + CD11c showed coexpression of hairy cells in 100% of our patients. The scoring system for hairy cell leukaemia used in our patients showed that high scores 3 and 4 had 97% of patients.