Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematologic disordr characterized by increased susceptibility of erythrocytes to complement-mediated hemolysis. Recently, PNH is a stem cell disorder with all hematopoietic lineages affected, which have deficiencies of glycosylphosphtidylinositol-anchored membrane proteins due to the phosphatidylinositol glycan-class A (PIG-A) gene abnormalities. However, it is unknown how PNH clones with PIG-A gene abnormalities increase in bone marrow. The possibility has been suggested that resistance of PNH cells to apoptosis causes the increase. We studied two-color or single-color flow cytometric analysis using Annexin V and propidium iodide or 7-amino actinomycin D for evaluation of spontaneous apoptosis in peripheral blood granulocytes from PNH patient (n = 5) and healthy volunteers (n = 5), respectively. Apoptotic granulocytes were evaluated before and after 6-, 12-, 18- and 24-hour cultures without serum. Flow cytometric analyses showed that there were no significant differences of the number of proportion of apoptotic cells between them. This fact reveals that the sensitivity of PNH cells to apoptosis is similar to that of normal cells, suggesting that PNH clones should not be increased according to resistance to apoptosis as an intrinsic characteristic.