To determine whether true obstruction to left ventricular ejection exists in patients with hypertrophic cardiomyopathy and a subaortic gradient, pulsed Doppler echocardiography was used to analyze the patterns of left ventricular emptying in 50 patients with hypertrophic cardiomyopathy (20 with and 30 without evidence of obstruction) and in 20 normal subjects. In obstructive hypertrophic cardiomyopathy, left ventricular ejection was characterized by early and rapid emptying (76 ± 14% of aortic flow velocity in the initial one-third of systole). The proportion of forward flow velocity occurring before initial mitral-septal contact (and hence, by inference before the onset of the subaortic gradient) was variable, but averaged 58%. In contrast, the proportion of forward flow velocity occurring after mitral-septal contact (and, therefore, concomitant with the gradient and increased intraventricular pressure) was considerable, averaging over 40%. Mid-systolic impedance to left ventricular outflow was suggested by the rapid deceleration in aortic flow velocity concomitant with mitral-septal contact and premature partial aortic valve closure. Furthermore, left ventricular ejection was prolonged (384 ± 40 ms) and the ventricle continued to empty and shorten during the period when both the pressure gradient and markedly increased intraventricular pressures were present. In 16 of 20 patients, a relatively small second peak in flow velocity appeared in late systole. Since marked systolic anterior motion of the mitral valve was still present, the late systolic portion of forward flow velocity also appeared to be largely ejected during imposition of a mechanical impediment to outflow. In contrast, patients with nonobstructive hypertrophic cardiomyopathy showed no evidence of impedance to left ventricular ejection. Aortic flow velocity waveforms were similar to those of normal subjects, with flow persisting to aortic valve closure; significant mitral systolic anterior motion and partial mid-systolic aortic valve closure were absent, and the systolic ejection period was normal (303 ± 27 ms). It is concluded that in hypertrophic cardiomyopathy: 1) systolic anterior motion of the mitral valve produces a mechanical obstruction to left ventricular emptying, and a considerable portion of the stroke volume is impeded in its egress from the left ventricle; 2) the subaortic gradient appears to be of pathophysiologic importance because the left ventricle continues to contract in the presence of markedly increased intraventricular pressures; and 3) left ventricular ejection characteristics differ markedly between patients with and without mitral systolic anterior motion or a subaortic gradient.