Between September 1986 and February 1989, ninety patients undergoing isolated aortic valve replacement were studied on the 15th postoperative day, in the Department of Cardiac Surgery at St Joseph's Hospital, Paris. Patients with a diastolic murmur, fever, significant, pericordial effusions and poor quality Doppler recordings were excluded. The aim of the study was to determine normal Doppler echocardiographic parameters of St Jude medical aortic valve prostheses. Maximum and mean transprosthetic pressure gradients were calculated by the simplified Bernoulli equation. Functional value surface area was assessed by the continuity equation using the diameter of the left ventricular output tract measured by 2D echocardiography (continuity) and the external diameter of the prostheses (modified surface area) to calculate the subaortic surface area. The transprosthetic pressure gradients G max and G mean were inversely correlated to the size of the prostheses (r = 0.42 and r = 0.45). The functional valve surface area calculated by the continuity method gave a poor correlation but this improved considerably when the diameter of the prosthesis (modified surface area) was considered. Finally, the permeability index which is the ratio of subaortic and transaortic velocities seemed to be a good parameter for assessing prosthetic valve function.