Twenty-one cases of mitral valve prolapse (MVP) diagnosed by M-mode echocardiograms were studied in regard to the underlying or associated conditions, types of MVP, and dimensions of the cardiac structures and parameters reflecting cardiac performance determined by echocardiograms. In 9 cases, MVP was thought to be idiopathic origin. Associated conditions in the remainder were 5 cases of congenital heart disease, 2 cases of rheumatic myocarditis, 1 case each of aortic valve disease, ischemic heart disease and hyperthyroidism, and 2 cases of undiagnosed cardiac conditions. There were 2 cases of buckling type of MVP and in the remainder hammock type. In these 2 patients with hammock type of MVP, mitral valve echoes were seen in the left atrium during systole in 1 case, and diastolic anterior displacement of posterior mitral leaflet in the other. In 9 cases of idiopathic MVP, cardiac dimensions and parameters reflecting cardiac performance as determined from echocardiograms showed no significant differences from those of 27 normal controls.