Granulocytes from PNH patients are relatively resistant to apoptosis as compared with granulocytes from normal controls. Cells were tested in triplicate and the mean is plotted for each patient (○). The mean for each group is shown as a horizontal line. The amount of apoptosis as measured by flow cytometry was significantly less for PNH granulocytes (38.8% ± 14.1%, n = 26) as compared with normal granulocytes (55.0% ± 12.0%, n = 20), P < .001. There was some overlap for individuals within each group.