A large surplus portion of the mitral leaflets appears to be involved in the systolic anterior motion (SAM) of the mitral valve in hypertrophic cardiomyopathy (HCM). To understand the exact features and the mechanism of the surplus leaflet, we studies the mitral valve and its ring quantitatively with two-dimensional echocardiography in 17 patients with HCM (5 with large SAM, 6 with small SAM and 6 without SAM) and 10 normal subjects. In the large SAM group, the surplus length of the anterior mitral leaflet (AML) increased but that of the posterior mitral leaflet (PML) did not increase when compared to the other 3 groups. In the large SAM group, the length of AML was greater than in the normal group and the length of PML was greater than in the other 3 groups. The mid-systolic anteroposterior diameter of the mitral ring did not differ among the 4 groups. The mid-systolic position of the coaptation was more anterior in the large SAM group than in the other 3 groups. In conclusion, the surplus size of AML is actually increased in the patients with HCM and large SAM, which is caused by the elongation of both AML and PML with anteriorly displaced coaptation and with no dilation of the mitral ring.