An important aspect of the management of patients with myasthenia gravis is the decision to recommend thymectomy. Hitherto, many investigators have reported the relationship between the operative effects and such factors as age, sex, duration of symptoms, or degree of germinal center proliferation in the myasthenic thymus. However, these reports are not practical aids in deciding the indication for thymectomy in an individual myasthenic patient. The currently accepted indications of thymectomy for myasthenic patients are (1) the thymomatous patient, especially those with malignancy, and (2) the nonthymomatous patients who are resistant to medical treatment. From our present data we would add the following as an indication of the operation: (3) patients who have high T-cell subpopulation levels with highly blastogenic activities and strong skin test reactivities. In order to assure good operative results in myasthenic patients, surgeons should examine their patients' preoperative immunological states.