Abstract： Background:Transesophageal echocardiography (TEE) is a well-established method for detecting and diagnosing heart tumors.In contrast,its role in assessing the presence,growth and evidence of malignant tumors originating from mediastinal sites remains unclear.The aim of this study was to compare the diagnostic impact of TEE and transthoracic echocardiography (TTE) for determining the localization,growth and malignancy of adult mediastinal tumors (MTs).Methods:In a prospective and investigator-blinded study,we evaluated 144 consecutive patients with MT lesions to assess the diagnostic impact of TEE and TTE for detecting the presence of tumors spreading both inside and outside of the heart and for determining infiltration and invasion using pathological examination results as a reference.Results:All tumor lesions were diagnosed and carefully evaluated by biopsy.Biopsy revealed malignant tumors in 79 patients and benign tumors in 65 patients.When compared to histological findings,TEE predicted malignancy from the presence of tumors spreading both inside and outside of the heart and from infiltration and invasion in 49/79 patients (62.0％).TTE predicted malignancy in only 8/79 patients (10.1％,P ＜ 0.005).TEE visualized tumor lesions in 130 patients (90.3％) while the TTE visualized tumor lesions in 110 patients (76.4％) and was less effective at detecting MT lesions (P ＜ 0.001).TTE and TEE could detect anterior MTs and adequately verified MTs (P ＞ 0.05);TEE detected medium MTs better than TTE (P ＜ 0.001).Conclusions:TEE is effective and superior to TTE for predicting the localization and growth of MTs as well as for accessing evidence of tumor malignancy.TTE and TEE were able to detect anterior MTs;TEE was able to detect medium MT better than TTE.