Paroxysmal nocturnal hemoglobinuria (PNH) is a disorder characterized by the deficiency of glycosyl phosphatidylinositol (GPI)-anchored proteins in blood cell membranes. We experienced a patient with PNH whose grade of hemolysis was reduced during the course of the disease. An analysis of the expression of GPI-anchored proteins on blood cells revealed typical PNH defects in polymorphonuclear leukocytes (PMN), but not in red blood cells (RBC). Accordingly, we investigated the expression of CD59 on RBC and of CD59 and CD16 on PMN in PNH patients, using immunofluorocytometry. We examined the cells of 8 PNH patients with different grades of hemolysis. PNH-affected PMN deficient in CD16 and CD59 were clearly demonstrated in every patient, including those with low-grade hemolysis. We conclude that demonstration of PNH-affected PMN deficient in GPI-anchored proteins has diagnostic values even in PNH patients with low-grade hemolysis.