A 53-year-old man with osteogenesis imperfecta underwent valve replacement and coronary artery bypass surgery. Unexplained symptoms of dyspnea and fatigue in the late postoperative period prompted further evaluation. Transthoracic echocardiography demonstrated obstruction of the right ventricular outflow tract and pulmonary artery by a mass, with a maximum gradient of 50 mm Hg. Multiplane transesophageal echocardiography revealed that the mediastinal hematoma was more extensive than was suggested by the transthoracic echocardiogram. This finding prompted the decision to resort to a more extensive surgical procedure. Surgical drainage continued until no residual hematoma could be visualized by multiplane transesophageal echocardiography. This case report demonstrates the value of multiplane transesophageal echocardiography in the assessment of mediastinal masses.