Fifty‐four patients with thymoma were seen at Memorial Hospital from 1928 to 1972. There were 18 benign and 36 malignant thymomas. Benign tumors were completely encapsulated. Malignant tumors were non‐encapsulated or incompletely encapsulated and invasive. All patients with benign thymoma were treated by resection alone. None had recurrent disease or died with tumor for periods ranging from 5 to 17 years after resection. Patients with malignant thymoma treated by resection alone had recurrent tumor within 5 years and subsequently died of their disease. All malignant thymoma patients who were alive free of disease for 5 or more years had radiation therapy and resection. Clinicopathologic features indicating a poor prognosis were: a non‐encapsulated tumor of a predominantly epithelial cell type, superior vena caval syndrome, malignant pleural effusion, supraclavicular lymph node involvement, dysphagia, hoarseness, myasthenia gravis, erythroid hypoplasia, and hypogammaglobulinemia.