The x-ray film of the chest, by giving clear evidence of incomplete closure of the disk valve, was diagnostic of prosthetic valve malfunction in two patients. Potentially lethal malfunction of a prosthetic disk valve can be treated successfully by operation if the diagnosis is made early in the clinical course. The medical history is helpful. The physical examination can lead to the diagnosis, but the results may be normal in the presence of valve malfunction. It may be possible to detect malfunction in the majority of instances by x-ray film of the chest and fluoroscopy, but if these studies are inconclusive when malfunction is suspected, cardiac catheterization and cineangiography should be done promptly.