A 78-year-old woman complaining of suddenly developed numbness and coldness of the left hand was referred to our hospital on the suspicion of embolism. A2x1cm mass was revealed in the f ossa ovalis of the interatrial septum by echocardiography. She underwent operation under a preoperative diagnosis of thrombus in the left atrium or tumor of the interatrial septum. In the operation, the mass was excised including the interatrial septum and the defect of the interatrial septum was closed with a

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... FE patch. A sagittal section of the mass showed that it was a fresh thrombus covered with normal endocardium of the fossa ovalis except for a small protrusion to the left atrial cavity. These findings yielded diagnosis of thrombus trapped in a patent foramen ovale. She was discharged after an uneventful postoperative course, although temporary pacing was needed for transient bradycardia in the early postoperative days. In this case, we hypothesize that an unidentified venous thrombus trapped in a patent foramen ovale had partly passed through the interatrial septum and caused paradoxical embolism in the left hand. Paradoxical embolism is a well-known phenomenon described in a number of reports. There are some reports of a trapped thrombus in a patent foramen ovale detected by echocardiography or in autopsy. We describe a rare case of surgical demonstration of a trapped thrombus in a patent foramen ovale, and recommend that examinations for venous thrombus with a suspicion of paradoxical embolism are necessary for patients of thromboembolism.