Clinical observation were performed on 25 cases of Mediastinal tumors or Cyst, those were admitted and treated at the Department of Thoracic and Cardiovascular Surgery, Korea University Hospital, during the 6 years period from March 1973 to March 1979. The following results are obtained. Of 25 cases, 19 patients were males and 6 patients were females. Range of age varied widely from 2 years to 72 years. Approximately 28% were younger than 15 years of age at the time of diagnosis. The common subjective symptoms of the patients were anterior chest pain [36%], coughing [27%], dyspnea and a few incidence of hemoptysis. Diagnostic procedures were posteroanterior and lateral chest roentgenorgrams, Chest tomograms, Brochograms, Esophagograms, Mediastinoscopy, Scalene and Axillary Lymph node biopsy, and Needle aspiration biopsy. In the histological distribution on Mediastinal tumors in order of frequency, Neurogenic tumor 6 cases [25%], Lymphoma 5 cases [21%], Bronchogenic cyst 4 cases [17%], Pericardial cyst 2 cases [8.3%], Teratodermoid tumor 2 cases [8.3%], and each one case of Rhabdomyosarcoma, Seminoma, Cavernous hemangioma, Anthracosis, Tuberculoma were noted respectively. Of 24 cases of the histologically confirmed Mediastinal tumors, 6 cases [24%] were malignant. Thoracotomy for removal of tumor or cyst was performed on 17 cases and offered cure of all benign tumors. In 6 cases of malignant tumors, Chemotherapy with Vincristine, Cyclophosphamide and Prednisolone was given to 1 case Lymphoma. There was no case of postoperative mortality.[KTCS 1979;4:424-428]