Isotope angiocardiography established prosthetic tricuspid valve obstruction in a patient 6 years after triple valve replacement. Thrombus formation is a common complication of prosthetic cardiac valves, particularly in the tricuspid position where engagement of cage with myocardium is an additional potentially obstructive hazard. Comparison is made between the information yield of the isotope investigation and cardiac catheterization with contrast cineangiography. It is suggested that the isotope technique can noninvasively provide valuable anatomic and functional data, particularly where the right side of the heart is to be examined.