Abstract: The experience with 100 primary tumors and cysts of the mediastinum during a period of 20 years is presented. A total of 57 male and 43 female patients were included in the study. The age ranged from 1 1/2 to 65 years. The most frequent lesions were thymomas, neurogenic tumors and teratomas, being 27, 26 and 26 respectively. The others were cysts （9）, mesenchymal tumors （8）, intrathoracic goitres （3） and primary carcinoma （1）. There were 93 benign and 7 malignant lesions. About one-third of the patients were asymptomatic and were discovered by routine chest X-ray examination. In 68 patients with symptoms the common complaints were chest pain, cough, dyspnoea and myasthenia gravis. Plain postero-anterior and lateral chest roentgenograms revealed the actual mediastinal masses in the majority of the patients. So they were the main diagnostic measures in our patients. Early operative intervention is mandatory because of the symptoms they produced by the lesion and the possibility of malignancy. All of the patients in this series underwent surgery, including complete resection in 95 cases, partial resection in 4 and exploration alone in one. The operative mortality rate and postoperative morbidity were 1% and 6% respectively. In order to make surgery safe and successful, following principles are recommended: （1） Preoperative diagnosis should be made accurately, and ectopic kidney, aortic aneurysms, etc, should not be mistaken for mediastinal tumors. （2） The operative approach must be appropriate for each individua lesion. （3） Operative manipulation should be prudent and careful. （4） It must be borne in mind that some of the mesenchymal tumors may interpose in between adjacent important structures, making total excision impossible.