To study the mechanism of altered mitral function in the presence of an isolated cleft mitral valve (ICMV) with regard to the relative roles of the cleft and of the accessory chordae, seven patients with ICMV were studied with color Doppler echocardiography. Mitral insufficiency ranging from mild to severe was demonstrated in six cases. The regurgitant jet originated in each case from the site of the cleft: in five patients the regurgitant jet had a narrow base originating exactly from the cleft; in the sixth patient, the regurgitant flow presented as a broad-based jet suggesting that accessory chordae restricted the motion of the anterior mitral leaflet. Turbulent flow in the left ventricular outflow tract, starting at the level of the accessory chordae, was found in one patient in whom a pressure drop of 44 mm Hg was detected with continuous-wave Doppler imaging. The altered function of the mitral valve cleft stems from two elements, the cleft itself and the accessory chordae. Color Doppler flow imaging showed that the cleft was the main factor causing mitral insufficiency. The accessory chordae played an additional pathogenetic role in two patients by causing restricted mitral motion or left ventricular outflow tract obstruction.