The aims of this study were to assess the clinical characteristics of myasthenic crisis after thymectomy (MCAT) and to identify predictors affecting the occurrence of MCAT. Of 66 patients with myasthenia gravis (MG), MCAT occurred in 20 patients (30.3%). The median time interval from thymectomy to MCAT was 3.4 months. MCAT occurred in 65.0% of patients within the first 6 months of a thymectomy, and 35.0% after 6 months. A second MCAT occurred in nine (45.0%) patients who survived the first MCAT, and in seven (50.0%) of 14 patients with a history of a preoperative myasthenic crisis before thymectomy (MCBT). A history of MCBT, and clinical factors reflecting perioperative clinical severity at thymectomy, including preoperative Osserman's grade, bulbar symptoms, use of immunosuppressants, pulmonary function, and postoperative delayed ventilator weaning, were significantly correlated with the occurrence of MCAT on univariate analysis. However, a history of MCBT was the only independent factor affecting the occurrence of MCAT on multivariate logistic regression analysis (odds ratio, 17.9; 95% confidence interval, 4.019-79.873; p<0.001). Thus, the occurrence of MCAT may be correlated only with a history of MCBT rather than with factors reflecting perioperative clinical severity. MG patients with a history of MCBT are more susceptible to MCAT, particularly within the first 6 months of thymectomy.