A 52-year-old man was given emergency admission because of progressive facial edema and dyspnea. Chest X-ray film on admission showed a large hemispherical mass in the right paratracheal region. Chest CT scan showed a large tumor in the mediastinum, narrowing of the trachea and superior vena cava. Severe snoring at night and excessive daytime sleepiness were noticed. Obstructive sleep apnea syndrome (OSAS) was diagnosed by polysomnogram (PSG). Although a histological diagnosis of the tumor was not established, chest irradiation was begun to reduce the size of tumor. After treatment, the mediastinal tumor almost completely disappeared, and the facial swelling and snoring were gradually relieved. PSG showed remarkable improvement of the apnea-hypopnea index (AHI). These findings suggested a significant correlation between SVCS and OSAS. After three months, right inguinal lymphadenectomy was done and it was diagnosed as malignant lymphoma (mantle cell lymphoma). OSAS may cause a deterioration of respiratory symptoms and the quality of life in a patient with SVCS. Therefore, in the case of SVCS, it is important to take account of the presence of OSAS and select an appropriate treatment.