In a 71-year-old man, acute myelomonocytic leukemia developed six years after a diagnosis of paroxysmal nocturnal hemoglobinuria (PNH) had been established. The classic features of PNH disappeared with the onset of the leukemia. Chemotherapy resulted in complete leukemic remission, during which time intravascular hemolysis and a positive acidified serum (Ham's) test recurred; both findings again disappeared when the leukemia recurred. To our knowledge, this is the eighth reported case of PNH terminating in acute leukemia but is the first in which reappearance of PNH has been documented with leukemic remission. The alternating pattern of the two disorders suggests that the PNH clone survivors in the bone marrow when leukemia supervenes.