Lymphoproliferative disease of granular lymphocytes (LDGL) is a disorder characterized by the clonal expansion of granular lymphocytes. It has recently been shown that the clonal expansion of granular lymphocytes occurs in patients with paroxysmal nocturnal hemoglobinuria (PNH) in a subclinical fashion. To test the possibility that LDGL patients share a PNH phenotype, we obtained peripheral blood cells from 20 patients with LDGL and examined the expression of the glycosylphosphatidyl inositol (GPI)-anchored proteins, CD55 and CD59. Compared with normal controls, however, a defective expression of CD55/59 was not observed on either granulocytes or erythrocytes from LDGL patients. An unexpected finding was the significantly lower CD55/59 expression on granular lymphocytes from patients with CD16(+)CD56(-) phenotype LDGL than from patients with CD16(+)CD56(+) phenotype LDGL, or natural killer (NK) and NK/T lymphocytes from healthy individuals. The positive correlation between the expression of CD56 and CD55/59 might have some relevance to the functional properties of the CD56(+) subset of large granular lymphocytes.