Two patients with choreatic syndromes caused by polycythemia vera recovered after treatment of the polycythemia by only two venesections: this proves that the syndrome is due to reversible alterations. Investigations of the cerebral circulation in one of the patients showed that blood flow was lowest in the grey matter at the basal region of the brain: this suggests that the alterations might mainly occur there. However, investigation of erythrocyte rheology, glucoses-phosphate dehydrogenase, serum concentrations of caeruloplasmin and serotonin, and urinary excretion of epinephrine, norepinephrine and vanillylmandehc acid gave normal results in both patients. There are therefore no indications as to the possible pathophysiology of these alterations. There are now 24 cases reported, including our 2 patients, which suggests that the association of these two diseases may not be so rare as supposed.