The occurrence of uncontrolled and purposeless movements in childhood involving primarily the extremities but affecting in severer grades voluntary muscles elsewhere, and for which there exists no demonstrable neurologic or psychologic background, is accepted as so-called Sydenham's chorea. The close association between this symptom complex, known as chorea, and rheumatic fever has been recognized for many years. Not only the frequency of choreiform movements in patients exhibiting symptoms and signs of rheumatic fever but also the high familial incidence of the two, together with the similarity in seasonal variation to be commented on later, is sufficiently striking to suggest a close interrelation between these two conditions. Furthermore, the frequent association between chorea and heart disease of the rheumatic type lends additional supporting evidence that Sydenham's chorea is probably a manifestation of rheumatic fever. Proof of this relationship, however, must await the establishment of the etiology of the two. It is