In a case of paroxysmal nocturnal hemoglobinuria, documented since 1955, the features of acute leukemia became evident in 1962, leading to quick death. Between 1960 and 1962, indications of PNH became greatly diminished and a slight, nonspecific leukocytosis developed. No apparent etiologic agent either for the PNH or the acute leukemia could be elicited.