A 75-year-old woman with a history of atrial fibrillation, severe biatrial enlargement, persistent dense spontaneous echo contrast in the left atrium and left-atrial appendage on appropriate oral anticoagulant therapy underwent implantation of the left-atrial occlusion device to decrease her risk of thromboembolic stroke. Six weeks later, a scheduled transesophageal echocardiography showed a partially mobile mass (4.1 × 2.1 cm) overlying the atrial surface of the device: a suspected thrombus. This report highlights the importance of follow-up examination and the limitation of this treatment modality among patients with persistent atrial fibrillation, large left atria and dense spontaneous echo contrast.