Fifty-five patients with primary mediastinal tumors and cysts which were seen at T.S. Dept., HYUH, were analyzed clinically, histologically, in an incidence of anatomic location and therapeutic results. The results were summarized as follows; 1] The ages of the patients in this series ranged from 15 months to 79 years with the highest incidence in the age group of third decade, and there were no sex distribution[M:F=0.96]. 2] The most frequently encountered tumors were teratodermoid tumors[29%] followed by neurogenic tumors[22%], thymomas[15%] and benign cysts[11%] in decreasing order of frequency. 3] Based on the subdivision of the mediastinum, 62% of the tumors were in the antero-superior mediastinum, 7% in the middle mediastinum and 31% in the posterior mediastinum. 4] The most frequent symptom was chest pain and others were dyspnea, cough, chest tightness and dysphagia. Asymptomatic patients were 29%. 5] Benign tumors and cysts were 71% and malignant tumors were 29%. 6] The successful removal was possible in all the benign mediastinal tumors and cysts[39 cases] and partial removal or biopsy was performed in the 12 cases among 16 cases of malignant mediastinal tumors. 7] Postoperative complications were bleeding, chylothorax, vocal cord paralysis, wound infection and hypothyroidism. 8] The most frequent mediastinal tumor in the West is neurogenic tumor but is teratoma in Korea.