There has been debate on the importance and pathophysiologic effects of the dynamic subaortic pressure gradient in hypertrophic obstructive cardiomyopathy. The study was conducted to elucidate the hemodynamic abnormalities associated with the dynamic pressure gradient in hypertrophic obstructive cardiomyopathy.Eight patients with hypertrophic obstructive cardiomyopathy and 7 patients with valvular aortic stenosis underwent a detailed hemodynamic study of pressure flow relationships before and after myectomy or aortic valve replacement during operation.In aortic stenosis, the increased gradient after premature ventricular contraction was associated with an increase in peak flow (325 ± 122 mL/s to 428 ± 147 mL/s, P = .002) and stroke volume (75.0 ± 27.3 mL to 88.0 ± 24.0 mL, P = .004), but in hypertrophic obstructive cardiomyopathy peak flow remained unchanged (289 ± 79 mL/s to 299 ± 85 mL/s, P = .334) and stroke volume decreased (45.9 ± 18.7 mL to 38.4 ± 14.4 mL, P = .04) on the postpremature ventricular contraction beat. After myectomy, the capacity to augment stroke volume on the postpremature ventricular contraction beats was restored in patients with hypertrophic obstructive cardiomyopathy (45.6 ± 14.4 mL to 54.4 ± 11.8 mL, P = .002).The pressure flow relationship in hypertrophic obstructive cardiomyopathy supports the concept of true obstruction to outflow, with a low but continued flow during late systole, when the ventricular-aortic pressure gradient is the highest. Septal myectomy can abolish obstruction and restore the ability to augment stroke volume, which may explain the mechanism of symptomatic improvement after operation.