We made a retrospective assessment of the long-term outcome in 247 consecutive patients with myasthenia gravis (MG) who underwent thymectomy in the period January 1971-December 1985. In 84 cases a thymoma was found at surgery, while 163 patients had a non-neoplastic thymus. The duration of symptoms before surgery, the age at onset of the disease and the presence of germinal centres in the thymus did not appear to influence the prognosis. Patients with a non-neoplastic thymus showed a better response to thymectomy. Thymoma was associated with more severe disease and with a higher mortality; moreover, more thymoma patients required corticosteroid treatment in order to achieve good therapeutic results. In our opinion, thymectomy is indicated in the treatment of generalized MG, while ocular myasthenia seems not to be improved by the removal of the thymus.