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A 63 year old woman with severe mitral regurgitation and permanent atrial fibrillation was referred for surgical replacement of the mitral valve. Transoesophageal echocardiography (TOE) performed at another facility suggested that this patient had an obvious thrombus in the left atrial appendage (LAA). TOE was repeated to evaluate whether surgical repair of the mitral valve was possible. Bileaflet prolapse of the mitral valve caused by myxomatous degeneration was seen. The left ventricular size and ejection fraction were normal; the left atrium was enlarged. Notably, there was no spontaneous echo contrast in the left atrium; the LAA velocity was mildly decreased. Evaluation of the LAA indicated the presence of a curious structure resembling a sea anemone exhibiting a “hand waving motion” (panel A: arrow indicates the mass located in the left atrial appendage. LA, left atrium; LV, left ventricle). Finger-like projections distinguished the latter as an atrial fibroelastoma. The LAA and fibroelastoma were resected during an uneventful surgery. Macroscopic examination and histological preparations confirmed the diagnosis; frond-like protrusions are clearly visible under microscopy (panel B). Although conventional wisdom would suggest that the structure in question was a thrombus, no echocardiographic data suggesting significant impairment of left atrial function were seen. Fibroelastomas occur primarily on valves, namely the mitral and aortic. They have been rarely described in the left atrial appendage, and may be confused with thrombi.