OBJECTIVE A few thymoma patients without myasthenia gravis (MG) have been observed to develop MG after total removal of the thymoma (postoperative MG). However, the cause of this is not yet known because of the rarity of postoperative MG patients. This study evaluated the clinical characteristics of the 8 postoperative MG patients. METHODS We compiled 1089 thymoma patients treated between 1990 and 1994 in 115 institutes in Japan, and found 8 cases of postoperative MG. RESULTS Postoperative MG was found in 8 (0.97%) of 827 thymoma patients without preoperative MG. The postoperative MG patients included 1 male and 7 females, with a mean age of 50.5+/-15.0 years. The thymoma was completely resected in all cases. The surgical method used was extended thymectomy in 2 cases and thymothymectomy in 6 cases. There were 2 cases (0.7%) of postoperative MG in the extended thymectomy group (n = 275), 6 (1.9%) in the thymothymectomy group (n = 321), and none in the tumor resection group (n = 137). The interval between thymectomy and the onset of postoperative MG varied (6 days-45 months, 19.3+/-16.5 months). The type of MG was ocular in 2 cases and general in 5 cases, according to the modified Osserman classification. The postoperative MG was responsive to anti-cholinesterase compounds and/or steroids. The improvement rate was 86%. CONCLUSIONS Postoperative MG was present in about 1% of the patients who underwent total thymoma resection. Resection of the thymus gland does not prevent postoperative MG.