Paroxysmal nocturnal hemoglobinuria is an acquired clonal stem cell disorder characterized by the decrease or absence of glycosylphosphatidylinositol-anchored molecules from the surface of the affected cells, such as CD55 and CD59, resulting in chronic intravascular hemolysis, cytopenia and increased tendency to thrombosis. PNH-phenotype has been described in various hematological disorders, mainly in aplastic anemia and myelodysplastic syndromes, while it has been reported that complete deficiency of CD55 and CD59 has also been found in patients with lymphoproliferative syndromes, like non-Hodgkin's lymphomas. The presence of CD55- and/or CD59-defective red cell populations was evaluated in 217 patients with lymphoproliferative syndromes. The study population included 87 patients with NHL, 55 with HD, 49 with CLL, 22 with ALL and four with hairy cell leukemia. One hundred and twenty-one healthy blood donors and seven patients with PNH were also studied as control groups. The sephacryl gel microtyping system was performed for the detection of CD55- and CD59-deficient red cell populations. Ham and sucrose lysis tests were also performed in all samples with CD55 or CD59 negative populations. Red cell populations deficient in both CD55 and CD59 molecules were detected in 9.2% of patients with lymphoproliferative syndromes (more often in ALL and nodular sclerosis type of HD) and in all PNH patients. CD55-deficient red cell populations were found in 8.7% of LPS patients (especially in low grade B-cell NHL), while CD59-deficient populations were found in only two patients with low grade B-cell NHL. These data indicate a possible association between paroxysmal nocturnal hemoglobinuria phenotype and lymphoproliferative syndromes, while further investigation is necessary to work out the mechanisms and the significance of the existence of this phenotype in these patients.