Catastrophic thrombosis of porcine aortic bioprostheses. Hemodynamically critical thrombotic stenosis of porcine bioprosthetic valves in the aortic position without thrombotic predisposition is rare. Two patients at our institution abruptly manifested thrombotic stenosis of porcine bioprosthetic valves in the aortic position within 3 months of implantation without apparent predisposing factors. Clinicians should consider this rare but catastrophic complication in the appropriate setting. 