SUMMARY Echocardiograms from patients with inferior myocardial infarction (IMI) and from patients with asymmetric septal hypertrophy (ASH) or idiopathic hypertrophic subaortic stenosis (IHSS) are sometimes similar, because in both, the ratio of thickness of the ventricular septum (VS) to the thickness of the posterobasal left ventricular free wall (LVFW), both measured in diastole, may be; 1.3. In order to determine if IMI could be differentiated from ASH/IHSS by the echocardiogram, 10 patients with IMI, 10 patients with ASH and 10 patients with IHSS were studied by echocardiography, left ventricular cineangiography and coronary arteriography. Neither the magnitude of the VS/LVFW nor the VS or the LVFW thicknesses alone distinguished patients with IMI from patients with ASH or IHSS; however, the groups could be distinguished by evaluating additional echocardiographic features which reflect different pathologic anatomy and function. The patients with IMI showed 1) minimal systolic thickening of the LVFW such that the LVFW in systole was less 13 mm; 2) an increase in the VS/LVFW during systole; 3) a ratio of the VS excursion in systole to the LVFW excursion _ 1.0; and 4) a ratio of VS to the systolic diameter of the left ventricular cavity (D,) - 0.5. The patients with either ASH or IHSS showed 1) systolic thickening of the LVFW such that the LVFW in systole was greater than 13 mm; 2) a decrease in VS/LVFW in systole; 3) a ratio of the excursion of the VS to the excursion of the LVFW < 1.0; and 4) a VS/D. > 0.5. These results show that some patients with IMI can resemble patients with ASH or IHSS because both have VS/LVFW (in diastole) 1.3. The disorders can be distinguished, however, by also considering the absolute systolic thickness of the LVFW, relative wall motions in systole, the VS/LVFW in systole and the VS as related to the D%. This differentiation is clinically important, since these disorders have different treatments and prognoses.