Twenty-three patients with the midsystolic click - late systolic murmur syndrome were investigated by right and left heart catheterization, cineangiography and echocardiography. Most had symptoms such as atypical chest pain and arrhythmias. Except in one patient, slight to moderate mitral incompetence was present at rest (20 patients) or during stress testing by the infusion of aramine or rapid atrial pacing (2 patients). In all cases, an abnormal systolic mitral valve motion was found by left ventricular cineangiography. In 14 of 15 technically satisfactory echocardiograms a systolic prolapse of one or both mitral leaflets was observed. A minority of the patients had localized abnormal wall motion in the posterobasal area or moderate generalized hypokinesis.