Attention is drawn to the association of polycythemia and focal cerebrovascular insufficiency; 22 patients known to have this combination of disorders have been observed at the Mayo Clinic in the years 1954 through 1957, and 10 of the cases are described in sufficient detail to illustrate the variety of problems that occur. It is emphasized that [1] the initial important phenomenon occurring in a patient with polycythemia can be an attack of insufficiency in the carotid or vertebral-basilar system; [2] It is necessary to examine the blood for polycythemia in every patient with focal cerebrovascular insufficiency; [3] the type of polycythemia should be determined, as specific therapy may be indicated (surgery for pulmonary arteriovenous fistula); [4] when focal cerebrovascular insufficiency occurs in patients known to have polycythemia, the polycythemia should be controlled as rapidly as possible; and [5] it may be necessary to administer anticoagulants for a variable period, depending on the frequency and severity of the episodes of focal cerebrovascular insufficiency. The concepts of the pathogenesis of intermittent insufficiency in the carotid and vertebral-basilar systems are briefly reviewed. The clinical evidence of polycythemia as a factor in pathogenesis is discussed.