In 33 thymic tumors myasthenia gravis was associated in 17. Half of these were invasive lesions, whereas only one fourth of the other 16 were invasive. Response of the myasthenia gravis to thymectomy was good to excellent in 44%. An average follow-up period of 5½ years revealed seven late deaths, five of them in patients with myasthenia gravis. Four of the seven late deaths were in patients with invasive tumors. In those patients with thymomas and associated myasthenia gravis, the response of the myasthenia to surgical excision of the thymoma is not predictable, although improvement can be expected in some. In addition, since approximately one half of these lesions are invasive, prompt exploration is essential if later invasion of vital structures by tumor is to be prevented. Postoperative radiation therapy is advised in all patients with invasive lesions, as well as in those with large, nonresectable tumors.