A 58-year-old woman presented with a 2-week history of acute dyspnea in the setting of subtherapeutic warfarin anticoagulation (international normalized ratio, 1.7 on admission) with a history of previous St. Jude bileaflet aortic valve replacement (AVR) 5 years previously. Transthoracic echocardiography revealed severe elevation of the prosthetic aortic transvalvular gradient (peak velocity, 5 m/s; peak gradient, 125 mm Hg) with severe eccentric aortic regurgitation. The morphology of the prosthetic aortic valve was not clearly visualized secondary to an acoustic shadow from the metallic prosthesis. The patient was referred to cardiac computed tomography (CT) for further evaluation of prosthetic dysfunction and to exclude coronary artery disease in case surgical intervention was required.Dual-source cardiac CT (Definition, Siemens Healthcare, Erlangen, Germany) was able to demonstrate the underlying cause of the prosthetic valve dysfunction. The AVR was obstructed by acute thrombus ( Figure 1A) with fixation of the posterior leaflet with an abnormal opening angle of 80° ( Figure 2A and Movies IA and IIA in the online-only Data Supplement). This contributed to the significantly elevated transvalvular gradient and eccentric aortic regurgitation. The anterior leaflet maintained normal mobility. No significant coronary artery disease was detected on cardiac CT angiography. Normally functioning St. Jude AVR should have a maximal opening angle of up to 10°in systole and a closure angle of 120°to 130°in diastole between the 2 leaflets.The patient received successful thrombolysis treatment with recombinant tissue-type plasminogen activator (75 mg over 3 hours) with favorable outcome. A repeat cardiac CT within 24 hours showed dissolution of the thrombus and return of normal prosthetic valve function with a full range of leaflet mobility ( Figures 1B and 2B and Movies IB and IIB in the online-only Data Supplement).