To assess the prosthetic aortic valve functions according to types and sizes of valves, the peak flow velocity was recorded by means of continuous wave Doppler echocardiography in 40 patients (age 45 +/- 15 years) with prosthetic aortic valves and in 25 normal subjects. Twenty-one patients had Björk-Shiley valves (1-18 months after replacement); 12 had St. Jude Medical valves (6-48 months after replacement); and seven had Carpentier-Edwards porcine xenografts (48-84 months after replacement). The peak blood flow velocity across the prosthetic valve was recorded at the left ventricular apex, the suprasternal notch, and the right parasternal border in the second intercostal space. The pressure gradient was derived from the peak flow velocity by means of the simplified Bernoulli equation (P = 4V2). The peak and mean flow velocities and the peak and mean pressure gradients were measured to evaluate the opening function of the prosthetic valves. The results were as follows; The peak flow velocities were recorded in 33 patients (83%) with aortic valve replacements. All four measurements were significantly greater in patients with prosthetic aortic valves than in normal subjects, but there was no significant difference according to the valve type. The opening function was less in patients with smaller valves than in those with larger ones. There was a clear correlation between opening function and valve size. We concluded that continuous wave Doppler echocardiography is a useful noninvasive method for evaluating the opening function of a prosthetic aortic valve.