We have studied neutrophil intravascular life span in six patients with paroxysmal nocturnal hemoglobinuria (PNH); four had normal neutrophil counts when studied and two were neutropenic. Five patients had enough circulating neutrophils to isolate for tests in vitro. Lysis of labeled neutrophils was greatly increased, compared to that of normal volunteers, when these neutrophils were incubated with acidified fresh serum as a source of active complement plus serum containing antineutrophil antibodies (from three different sources). Despite the in vitro lesion, however, each of these patients had a normal neutrophil intravascular life span as measured by the 32P- diisopropylfluorophosphate technique. One neutropenic patient, who had a normal neutrophil life span, had a shift of cells from the circulating to marginated pool of sufficient degree to cause the neutropenia. A second (severely) neutropenic patient was found to have developed extreme marrow hypoplasia, also explaining the neutropenia. Thus, in contrast to the shortened red cell life span, we have been unable to find a shortened neutrophil life span in PNH.