SUMMARY Although the postoperative hemodynamic and echocardiographic features of idiopathic hypertrophic subaortic stenosis have been studied, the expected consistent postoperative thinning of the interventricular septum has not been reported. In this study, the short-term effects of septal myectomy were evaluated in 16 patients. All patients were assessed with pre- and postoperative hemodynamic studies and M-mode echocardiograms, and six of the 16 patients had pre- and postoperative two-dimensional echocardiograms. The mean resting preoperative gradient of 74 mm Hg (range 10-190 mm Hg), which fell to a mean resting postoperative gradient of 8 mm Hg (range 0-25 mm Hg), was associated with decreased end-diastolic interventricular septal thickness at the midventricular level in 14 of 16 patients and at the subaortic level in 16 of 16 patients by M-mode echocardiography. The group also demonstrated changes in left ventricular outflow tract configuration and dimension, mitral valve systolic anterior motion, mitral E-Fo slope and left ventricular percent fractional shortening by both M-mode and two-dimensional studies. In the two patients who did not show midventricular septal thinning on M-mode echocardiography, the two-dimensional echocardiograms revealed that the area of myectomy extended only through the subaortic region and not down to the midventricular septum. Thus, we have observed consistent postmyectomy septal thinning at both the midventricular and subaortic levels by M-mode echo. By defining the geometry of the septal myectomy in vivo with two-dimensional echocardiography, we can better interpret M-mode studies and identify factors that influence echocardiographic visualization of the region of myectomy.