In a retrospective series of 86 patients with myasthenia gravis, the only factors predictive of improvement in muscular strength after transsternal thymectomy were preoperative severity of myasthenia (90% versus 54%, p = 0.0014) and HLA-B8 (79% versus 50%, p = 0.0060) in bivariable and multivariable analyses. Both factors were not interrelated (p = 0.824). The statistical effect of HLA-B8 was independent from preoperative severity of disease. Typing for HLA-B8 may thus be a valuable adjunct in predicting the benefit of thymectomy in myasthenia. The observation that an MHC class I allele is associated with clinical improvement after thymectomy suggests that the clinical course of myasthenia may be influenced by class I restricted T-cells.