Polycythemia vera is a disease of unknown etiology characterized by an abnormally increased red cell mass. Its symptoms and complications are due to plethora and to thrombotic and hemorrhagic phenomena within various organs. Thrombosis is attributed to the high platelet count and to circulatory stagnation due to increased blood viscosity. This report is concerned with cerebral hemodynamics and their alteration by treatment during the several stages of the disease. Report of Case A 48-year-old Negro woman entered the District of Columbia General Hospital on Dec. 20, 1954, with the chief complaint of "spitting up blood." One month prior to admission the patient had suddenly become aware of a sensation of fullness in her mouth and expectorated "about a pint" of bright red blood. In the course of a similar, second episode, one week prior to admission, she had brought up "about one cupful." Two weeks prior to admission she had