Many varieties of space-occupying lesions can occur in the mediastinum. During the twenty-three-year period 1966-1988, we had experienced the 151 cases of primary mediastinal tumors and cysts at the Thoracic k Cardiovascular Dept., CUMC. The most common histologic types were thymomas and germ cell tumors, followed by neurogenic tumors and lymphomas. Most non-neoplastic lesions were granulomatous lesions and cysts. The mediastinal tumors were often asymptomatic, the malignant tumors in 35.4%, the benign tumors and cysts in 72.8% of the cases. The most common symptom was pain, which occurred in one-fourth of the patients. The most useful diagnostic method was X-ray examination of the chest. However, a final diagnosis could usually be made only at operation. Nine malignant tumors[30.0%] were excised radically, 17[56.7%] palliatively and 4[13.3%] were only biopsied. Almost all benign tumors except tuberculous lymphadenitis were radically excised. Fourteen patients received postoperative radiation therapy and 17 received chemotherapy. The postoperative complications were developed in 15[11.7%] and hospital mortality was 0.8%.