A 33 year old woman developed left ventricular outflow obstruction due to a fibrous subvalvular membrane 2 years after insertion of a Hancock modified orifice, porcine aortic valve prosthesis. M-mode and two-dimensional echocardiographic studies failed to detect the membrane. Continuous wave Doppler study revealed a maximal velocity of 4.8 m/s and a calculated peak pressure gradient of 92 mm Hg between the left ventricle and aorta. This finding was in close agreement with the peak gradient of 100 mm Hg obtained at cardiac catheterization. Because of its ability to accurately measure valvular gradients, continuous wave Doppler ultrasound study appears to be a useful noninvasive technique in the evaluation of patients with suspected prosthetic valve dysfunction.