The present study was conducted in an attempt to clarify the extent to which the preoperative severity of myasthenia gravis and immunosuppression favor postoperative infection. A retrospective analysis was carried out on 125 consecutive patients who had undergone transsternal thymectomy for myasthenia gravis between 1976 and 1995. The preoperative severity of myasthenia was graded by a modified version of Osserman's classification. The incidence of postoperative pneumonia among patients with Osserman's class 1, class 2, class 3, and class 4 disease were 0%, 10%, 21%, and 44%, respectively, showing a marked increase with the preoperative severity of myasthenia; however, postoperative wound infection and mediastinitis were unrelated to the preoperative severity of myasthenia. With every increment in Osserman class, there was an appreciable, though insignificant, rise in the frequency of preoperative immunosuppression. There was no significant association between postoperative infection and preoperative immunosuppression. These findings indicate that a poor preoperative clinical status has a greater impact on the risk of postoperative pneumonia than immunosuppression, and therefore, every effort should be made to decrease the preoperative severity of myasthenia. Promoting the widespread use of plasmapheresis seems particularly important for this purpose.