IN POLYCYTHEMIA vera, symptoms referable to the head greatly outrank in incidence those relating to any other bodily system.1 Actual cerebral vascular lesions are, furthermore, not infrequent. These include arterial or venous thromboses and hemorrhage, which is either intracerebral or subarachnoid in location. The development of extreme headache, amblyopia, papilledema and increased intracranial pressure in a case of well-marked secondary polycythemia led to studies that are of interest from the standpoint of intracranial dynamics, and to the conclusion that extreme plethora may result in changes indistinguishable from those occurring in other types of increased intracranial pressure.Case ReportA. F. . . .