Introduction A tumor of the mediastinum is no longer considered a medical curiosity. It was once thought to be a lesion of obscurity and inaccessibility, but with the advent of routine and mass-survey chest x-rays and improved surgical techniques it has become a lesion of ready visibility and resectability. Since many of these lesions are potentially malignant or, by virtue of their size and position, may cause serious symptoms, a review of the salient features of mediastinal tumors in indicated. This report deals with 174 cases of mediastinal tumors or cysts. It is our belief that this is the largest single series reported from any one hospital. Methods and Materials These 174 mediastinal tumors or cysts were operated upon at the University Hospitals from 1939 to 1957. Only lesions which originated primarily from the mediastinum were included in the series. The ages ranged from 2 weeks to 81 years, and