Background: There have only been retrospective uncontrolled studies of the effect of thymectomy on Myasthenia gravis (MG); Most of them show a positive effect, but the indication for operation is under discussion. Objective: To study the effect of thymectomy in MG. Material and methods: Questionnaire and investi-gation of patient records from 38 patients treated with thymectomy at Riks-hospitalet with MG between 1980 and 2002, and still alive. Results: All symptoms showed a 10-45% reduction in number of patients experiencing the symptom. Employment status increased from 23% to 46%. On a visual analogue scale (VAS) ranging from -2 to 10, the subjective effect was scored 5,8, long term effect 6,1, and the satisfaction all in all 6,7. The average cholinesterase inhibitor dose was reduced by 75%. The average corticosteroid dose increased with 75%, but comparing the group of patients using corticosteroids before and those using it after thymectomy, the dose was reduced by 63%. One patient became symptom free and 71% of patients showed improvement. Histology of thymus influenced the result. No correlation was observed between outcome and antibody concen-tration, duration of disease, patient age and hyperplasi. Male patients and patients with normal thymus histology achieved the greatest improvement. Conclusions: Thymectomy is effective in MG. The least effect was recorded in patients with thymoma. Antibody concentration, duration of disease, hyperplasi, patient’s age and female gender didn’t influence the outcome. Male gender and normal histo-logy are potential predictors of positive outcome, and are together with thymoma indications for thymektomy.