The dynamic behavior of fixed LV outflow tract stenosis partly resembles that of OCM. To analyze their differences we studied basal and postextrasystolic (post-PVC) peak-to-peak LV aortic gradients, aortic systolic pressure, and pulse pressure in 14 OCM and in 36 pure VAS without two-dimensional echocardiographic findings of OCM. Fifteen mild VAS had basal gradients similar to those of OCM (39 +/- 17 mm Hg vs 24 +/- 16 mm Hg). Patients with OCM show a post-PVC gradient (109 +/- 41 mm Hg) similar to that of VAS (110 +/- 50 mm Hg). However, the latter were departing from much higher gradients (VAS 72 +/- 30 mm Hg vs OCM 24 +/- 16 mm Hg). Decrement of post-PVC aortic systolic pressure and pulse pressure were frequent in both groups, but decrement of pulse pressure greater than 5 mm Hg were more frequent in OCM. We concluded that (1) post-PVC increased aortic gradients and decreased aortic systolic pressure occurred in both VAS and OCM; (2) post-PVC decreased aortic pulse pressure might occur in VAS; and (3) association of post-PVC gradient increment greater than 75 percent and pulse pressure decrement greater than 5 mm Hg are strongly suggestive of OCM.