Pressure half time has been assumed to be a relatively flow-independent measure of orifice area, but it may also be influenced by atrial and ventricular factors. Pressure half time and peak left ventricular inflow velocity were measured by continuous wave Doppler ultrasound in 164 patients with normally functioning Carpentier-Edwards, Björk-Shiley, and Starr-Edwards mitral prostheses. Pressure half time was shorter in the Björk-Shiley than in the other value types and peak transmitral velocity was highest in the Starr-Edwards prostheses. These differences, however, were partly explained by coexistent differences in transmitral flow. Filling time accounted for 19% and stroke volume for 15% of the variance in pressure half time compared with only 5.6% for prosthetic design and 0.4% for annulus diameter when each of these variables was considered alone. The design of the prosthesis explained 18% of the variance in peak transmitral velocity, while cardiac output and annulus diameter did not contribute significantly. With Doppler ultrasound it is impossible to define reliable normal ranges for prosthetic function independently of atrial and ventricular function. Formulas for orifice area based on peak transmitral velocity and flow seem more likely to reflect the behaviour of normally functioning prostheses than those based on pressure half time.