Thymectomy was performed on 249 patients with myasthenia gravis between 1957 and 1981. During a follow-up period that ranged from 2 months to 24 years (mean 7.5 years), the remission rate for the entire group was 51 percent, and an additional 36 percent had improvement (87 percent benefited). In those 51 patients with thymoma, the remission rate was 37 percent, with 68 percent of the patients benefiting from operations. The remaining 198 patients fared better after the operation: 54 percent had remission and a total of 91 percent benefited. The female patients within this group had a 94 percent probability of benefit from operation. We conclude that most patients with myasthenia gravis will benefit from thymectomy, and that the improvement persists over an extended period of time in a high percentage of patients.