An analysis is presented of 35 cases of chorea as a symptom of polycythaemia. This analysis reveals: (a) whereas polycythaemia occurs predominantly in males (3:2), polycythaemic chorea (PC) occurs predominantly in females (5:2), at a real ratio of female:male = 4:1, the prevalence being 1-2.5% of polycythaemic patients; (b) PC manifests predominantly after the age of 50 (8 cases before, 27 after 50 years), making polycythaemia the first disorder to be considered in cases of so-called 'senile' chorea; (c) PC is generalised, with predominant involvement of faciolingual and brachial muscles, and associated with muscular hypotonia; (d) PC may last from periods of weeks to years, usually responds to haloperidol, venesection or 32P-treatment, but may persist, or recur with treatment, or remit spontaneously, and (e) no relationship exists between the choreatic syndrome and (the rare finding of) a small infarct in the caudate nucleus. The cause of the choreatic syndrome in polycythaemia is presumably to be explained as a neostriatal hyperviscosity syndrome producing venous stasis, reduced brain blood flow and impaired tissular O2/glucose metabolism. The state of dopaminergic hyperactivity is presumably enhanced by relatively increased neostriatal catecholestrogens. The hypothesis of polycythaemic excess of dopamine-laden platelets releasing excess of dopamine in the neostriatum needs to be confirmed by laboratory evidence of platelet counts.