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Background. Medical treatment for myasthenia gravis (MG) involves the use of anticholinesterase agents, immunosuppressive drugs, plasmapheresis, and gammaglobulin. However, these agents result in a complete clinical remission rate as low as 15%. As a consequence, thymectomy, preferably by transsternal approach, has become increasingly accepted as an efficacious procedure for MG, with reported complete clinical remission rates as high as 80%. Methods. We have the clinical records of 61 patients diagnosed with MG at La Paz University Hospital, Madrid, Spain, from January 1977 to December 1994. All patients underwent thymectomy. The purpose of this investigation was to determine the major prognostic factors predicting MG outcome after operation. Results. Our results indicate that patients with a length of the disease from onset to operation shorter than 8 months have the best prognosis. Ossermann stages I and III are also associated with higher complete clinical remission rates. In contrast, neither age nor sex were found to be significantly related to MG outcome after thymectomy, although female patients have better prognosis than men, and the younger the patient the more likely is complete clinical remission. Pathologic findings after the operation were not found to be of prognostic value either. Conclusions. We conclude that thymectomy is a beneficial procedure for MG patients, with a complete clinical remission rate of 46% at 5 years postoperatively in our series. Therefore we advocate thymectomy for MG patients as early as possible in the course of disease because time elapsed from diagnosis to operation is the main determinant of the outcome.