Dual echocardiogram was recorded simultaneously with phonocardiogram (PCG) to analyze the isovolumic relaxation time (IRT) of the left ventricle (LV) in 85 patients with various heart diseases and in 23 normal subjects. The measurements used were time intervals from the onset of the aortic component of the second heart sound (IIA) to the onset of the posterior deflection of the posterior aortic wall in the aortic root echocardiogram (X) (IIA-X interval), and to the onset of the most rapid opening motion of the anterior mitral leaflet (D') (IIA-D' interval) during early diastole. 1. The IIA-X interval was directly proportional to the IIA-D' interval in the entire study population. 2. The IIA-X interval was prolonged with advancing age in normal subjects. 3. The IIA-X interval was significantly increased in patients with hypertensive heart disease, old myocardial infarction, hypertrophic cardiomyopathy, and dilated cardiomyopathy, but significantly decreased in patients with mitral stenosis. Thus, the IIA-X interval, which was measured easily and noninvasively from the aortic root echocardiogram is a reliable indicator of the isovolumic relaxation time of the left ventricle, as well as of the IIA-D' interval.