We describe a case of mediastinal angioleiomyoma in an asymptomatic 72-year-old man, who was admitted to our hospital for a mediastinal tumor discovered during an annual medical examination. The tumor was evaluated by computed tomography (CT) and magnetic resonance imaging (MRI). Unenhanced CT scans demonstrated a tumor that was adjacent to the descending aorta. The tumor was partially enhanced in the early phase of contrast-enhanced CT, and in the late phase there was additional tumor enhancement. With MRI, the tumor displayed a homogeneous low signal intensity on the T1-weighted image and a homogeneous very high signal intensity on the T2-weighted image. Contrast-enhanced MRI demonstrated the same enhancement pattern as CT. The examination results led to a preoperative diagnosis of posterior mediastinal hemangioma, and the patient underwent surgery. The tumor originated from the supreme intercostal vein, and was diagnosed as an angioleiomyoma by histopathologic examination. Because mediastinal angioleiomyomas are very rare, they are difficult to diagnose preoperatively. However, we believe that CT and MRI can be of significant help in the differential diagnosis.