We aimed to evaluate the clinical characteristics of patients with postoperative myasthenia gravis (MG). We retrospectively studied the data of 174 thymoma patients treated between 1990 and 2008 in Xiangya Hospital. Six of 125 patients without preoperative MG (4.8%) developed postoperative MG. The anti-acetylcholine-receptor binding antibody (ARAb) titers were elevated preoperatively in 22 of the 125 patients (17.6%) who did not have preoperative MG (range, 0.5-67.6nmol/L). Four of six patients with postoperative MG had positive ARAb levels preoperatively. Serum titers were exacerbated in all six patients at the onset of postoperative MG. Postoperative MG was responsive to anti-cholinesterase compounds and/or steroids. We concluded that a thymectomy did not prevent postoperative MG. Exacerbated ARAb levels after thymectomy suggested an extrathymic production of ARAb. We suggest that a rise in the ARAb titer might be a risk indictor for post-thymectomy MG.