Ten patients with systolic clicks and late systolic murmurs, documented phonocardiographically, were studied with the use of reflected ultrasound. In five patients, the anterior leaflet of the mitral valve displayed an abrupt posterior motion in mid-systole, coincident with the systolic click and onset of the systolic murmur. A similar motion of the posterior leaflet of the mitral valve was recorded definitely in one patient and possibly in three others; a visible separation of the mitral-valve leaflets in mid-systole was thus produced. In only one of the 10 patients did the recorded mitral-valve motion appear normal. Eight additional patients with systolic clicks but no murmurs were studied; in none of these could a posterior displacement of either valve leaflet in systole be detected. These findings support the cineangiographic observation that the syndrome of systolic click with late systolic murmur is a result of systolic prolapse of one or both mitral-valve leaflets into the left atrium. The echocardiographic pattern observed appears to be characteristic of this condition.