We report a case of a 63-year-old man without a history of atrial
fibrillation or mitral valve disease who was admitted to our hospital.
Echocardiography revealed a large left atrial mass attached to the
atrial septum. We suspected the mass to be a myxoma, but it turned out
to be a large thrombus after the surgery. Left atrial thrombus without
mitral valve disease or atrial fibrillation is rare. Although the degree
of urgency varies based on the case, early surgical resection is
recommended for a large left atrial thrombus to prevent embolism.