Previous reports have suggested that atrioventricular (AV) flow disturbances accompanying atrial myxomas mimic mitral stenosis. Two patients complaining of orthostatic syncope and positional intolerance had a large right and left atrial myxoma, respectively. Doppler flow records showed abrupt early diastolic flow cessation and normal velocity half-times, unlike AV valve stenosis. Large, obstructing atrial myxomas may behave as ball valves.