To determine whether the flow characteristics of aortic and mitral St. Jude Medical valves could be defined noninvasively, we analyzed Doppler transprosthetic flow velocity spectra in 23 relatively asymptomatic patients. Results were interpreted in the framework of M mode and two-dimensional echocardiographic data and were compared with Doppler transvalvular flow velocity spectra from native valves of healthy subjects. Although the morphologic characteristics of Doppler spectra were similar, peak and mean transprosthetic mitral flow velocities were higher than values obtained across native valves (1.38 ± 0.3 m/sec and 0.73 ± 0.1 m/sec vs 0.78 ± 0.1 m/sec and 0.35 ± 0.06 m/sec, respectively; p < .001). However, calculated pressure half-times were not different (61.2 ± 16.9 msec vs 57.2 ± 13.2 msec; p > .05) and calculated transprosthetic mitral gradients were small (2.3 ± 0.9 mm Hg). Similarly, the morphologic characteristics of aortic Doppler flow spectra in St. Jude and native valves were analogous. However, prosthetic valves exhibited higher peak and mean velocities (p < .01) and slightly prolonged time-to-peak flow (p = .02). M modes and two-dimensional studies did not show useful quantitative measures of prosthetic function and did not demonstrate evidence of paravalvular leaks, which were detected in four cases by Doppler techniques. Thus Doppler echocardiography provides quantitative information about transprosthetic flow characteristics in patients with implanted St. Jude valves and is useful in identifying patients with prosthetic dysfunction.