Eighty-three patients with myasthenia gravis underwent thymectomy were reviewed to evaluate the long term effects of thymectomy and analyse the factors influencing outcome. Follow up times were from 3 months to 120 months (mean: 77.6 months) after operation. The effects of thymectomy were as follows: 1) Remission rate was 26.5%, improvement rate was 56.5% and palliation rate was 83.1% at the last observed time in all cases. 2) Remission rates showed increasing (7.3% at 1 year, 22.2% at 5 years, 35.3% at 10 years) according to the time courses. Improvement rates and palliation rates showed respectively maximum (78.3%, 94.4%) at 4 years and 5 years. 3) Analysis of factors influencing outcome showed that the cases of IIA in Osserman's classification, less than 40 years old in age, less than 12 months in duration of illness, no complication of thymoma were expected the better effectiveness of thymectomy. We concluded that thymectomy for myasthenia gravis expected the delayed remission according to the time courses, and so, we must carefully follow the patients for long time.