The clinical and pathologic features of 283 patients with thymoma treated at the Mayo Clinic (147 female and 136 male; ages 16 years to 90 years; mean, 52 years) were examined. Forty-six percent of the patients had myasthenia gravis and 10% had other paraneoplastic phenomena. The tumors were locally invasive at operation in 32%, including 6% with metastasis to lung or pleura. Intrathoracic recurrence was noted postoperatively in 15% of those who had total excision and distant metastasis developed in 3% of patients. Thirteen percent died of their thymomas and 16% died of myasthenia. Overall 5-year survival was 67% and 10-year survival was 53%. Poor prognostic factors included presence of tumor-related symptoms, large tumor size, local invasion or metastasis in initial operation, and predominantly epithelial histologic features. Although true thymomas are composed of cytologically benign elements, they show a propensity for local invasion and intrathoracic recurrence. They rarely metastasize outside the thorax.