Reflux opacification of the coronary sinus was observed in 25 of 100 cases examined by cineangiocardiography. Of the 25 coronary sinus "positive" cases, 88 per cent were found to have cardiovascular lesions which are regularly associated with severe hypertension of the right heart chambers. The x-ray anatomy of the opacified sinus in different projections, and the movements by which it can be identified on x-ray motion picture films, are described in detail. The meaning of the reflux is discussed in relation to the diagnostic findings.