The commonest complications of polycythemia vera are those of the vascular tree. The frequency of thrombosis is generally recognized, and it is demonstrable that venous thrombosis is present in the majority of cases of polycythemia vera when they are first seen. The high incidence of thrombosis seems to be adequately explained by the huge increase in the volume of circulating blood of high viscosity. After the disease has persisted for a time, sluggish circulation, with chronic passive congestion, becomes exaggerated. The rate of circulation, as tested by the histamine method, when measured from leg to face or arm to face, is from five to ten times slower than that in the normal subject. There are changes in the blood, also, which favor thrombosis; high viscosity, hypercalcemia, and an increase in the number of platelets doubtless are important.
The arterial diseases associated with polycythemia vera have not been extensively studied. An