Mesothelial/monocytic incidental cardiac excrescence (cardiac MICE) is a rare intracardiac lesion, which can cause severe disease and lead to misdiagnosis.A 66-year-old woman with floppy mitral valve and coronary atherosclerosis underwent diagnostic coronary angiography, mitral valve surgery and aortocoronary bypass. During surgery, a thrombus was detected inside the left atrium and sent for pathology analysis. The sample was routinely processed, stained with histochemical dyes and immunomarked for epithelial (keratin) and mesenchymal cells (vimentin), histiocytes (lysozyme) and endothelial cells (Factor VIII).Histologically, the lesion contained mesothelial cells (keratin/vimentin coexpression), histiocytes (lysozyme/vimentin positive) and fibrin, leading to a diagnosis of MICE.Cardiac MICE is a benign cardiac tumor-like lesion. It may be misdiagnosed as a thrombus or neoplasia, either primary or metastatic. Through embolization, it may cause myocardial or multiorgan infarctions, in some cases fatal. Developments in cardiothoracic surgery and interventional cardiology may increase its incidence; hence the importance of recognizing this entity.