A case of a 51-year-old male with dilated cardiomyopathy and left atrial mass detected by echocardiography is described. Differential diagnosis between thrombus and myoxoma was difficult. Anticoagulation therapy was initiated and no intra-atrial mass was detected during the control echocardiographic examination. Thus, the diagnosis of intracardiac thrombus was established ex iuvantibus. Difficulties in the diagnosis and treatment of intracardiac masses are discussed.