Two-dimensional echocardiographic examination was performed to investigate pathophysiological mechanism of mitral regurgitation due to papillary muscle dysfunction in 6 patients with myocardial infarction (anterior : 3, inferior: 2 and subendocardial: 1) and 13 patients with dilated cardiomyopathy. The patients were divided into 2 groups; Group I was 9 with mitral valve prolapse and Group II was 10 without mitral valve prolapse (MVP). In Group I MVP was observed in the posterior leaflet in 6 and in both leaflets in the remainder. The apical four-chamber view of all patients in Group II showed that the point of mitral valve coaptation displaced toward the apex of the left ventricle during systole and the both mitral leaflets were convexed toward the left ventricle. Follow-up observation in a patient with dilated cardiomyopathy revealed that the heart was not enlarged (CTR 50%), no murmur was heard and mild anterior mitral leaflet prolapse was observed in 1979, but, in 1981, CTR became 63%, and a grade IV systolic murmur suggesting mitral regurgitation was noted. Two-dimensional echocardiography revealed that mitral leaflets became convex toward the left ventricle and coaptation point of the mitral closure shifted to the apex of the left ventricle below the level of mitral annulus. Thus, the transition to Group II from Group I was suggested. Echocardiographic measurements revealed that left ventricular end-diastolic dimension was 71 +/- 11 mm in Group I and 74 +/- 10 mm in Group II, left ventricular end-systolic dimension was 61 +/- 11 mm in Group I and 62 +/- 11 mm in Group II, and anterio-posterior mitral ring dimension was 31 +/- 3 mm in Group I and 35 +/- 3 mm in Group II (p less than 0.05). In conclusion, it was suggested that two mechanisms of mitral regurgitation are present in papillary muscle dysfunction; 1) mitral valve prolapse and 2) shift of mitral coaptation point toward the apex of the left ventricle associated with marked dilatation of the mitral annulus.