A new, noninvasive method for evaluating the in vivo function of the Bjork-Shiley tilting disc valve in the aortic mitral, and tricuspid positions was described. The tilting disc was equipped with a ring-shaped radiopaque marker, which permits visualization of the motion of the disc by both cineradiographic and fluoroscopic studies. A beam direction corresponding to the axis of motion of the disc should be sought for calculation of the prosthetic opening angle, whereas prosthetic valve closure is demonstrated by a beam direction deviating 20-30 degrees from the valve ring plane. Complete opening to 60 degrees and closure of the prosthetic valve, indicating free motility of the tilting disc, were confirmed by cineradiograms. The error of estimation of the in vivo opening angle is small with deviations of less than 15 degrees from the ideal beam direction and negligible for opening angles approaching 60 degrees. Deviations in 2 planes simultaneously cause less error than deviation in 1 plane. The radiopaque marker was designed to detect mechanical valve failure owing to thrombotic obstruction. The incidence of this particular complication was 8.1 per 100 patient-years after aortic valve replacement without anticoagulation treatment. With anticoagulation, this incidence was zero after aortic and 2.5 after mitral valve replacement. In the clinical experience to date, which comprises over 1250 implants, a single case of primary mechanical valve dysfunction was not encountered. The current program includes a change-over to the radiopaque tilting-disc model of the Bjork-Shiley prosthesis, which was introduced at this clinic in March, 1975.