Although transthoracic two-dimensional echocardiography has been a procedure of choice for diagnosing cardiac mass lesions, the advent of transesophageal echocardiography (TEE) provided better visualization of cardiac structures, especially those at a considerable depth from the chest wall, and lesions that involve the left atrial appendage. In this study, we examined the experience at our institution with TEE imaging of cardiac mass lesions (excluding valvular vegetations) from April 1988 to July 1990. TEE studies detected 83 lesions (in 80 patients), which we characterized by type and site: 46 left atrial, 16 right atrial, 7 left ventricular, 2 right ventricular, and 12 extracardiac mass lesions. Of the 46 left atrial lesions, 9 were tumors and 37 were thrombi that involved the body of the left atrium, the left atrial appendage, or both. Associated mitral valve disease, chronic atrial fibrillation, or spontaneous microcavitations were common. Of the 16 right atrial mass lesions, 4 were tumors and 12 were thrombi, including "string" thrombi characteristic of venous thromboembolism. Of the seven left ventricular mass lesions, six were thrombi and one was a papilloma. Of the 12 extracardiac mass lesions, 2 were pericardial cysts and the rest were solid lesions. TEE added new or important clinical information beyond that derived from transthoracic echocardiography in left atrial thrombi, right atrial masses, and extracardiac lesions and was assessed to have influenced the management of patients most in these areas also. TEE is a useful addition to transthoracic echocardiography for diagnosis and clarification of cardiac mass lesions in selected patients.