Good syndrome, characterized by both thymoma and hypogammaglobulinemia, is a rare immunodeficient disorder. We experienced a case of Good syndrome accompanied by myasthenia gravis (MG). A 58-year-old man was admitted to our hospital because of muscle weakness and fatigability. Based on the presence of anti-acetylcholine receptor (AChR) antibody and thymoma, he was diagnosed as having MG. Peripheral blood lymphocyte count was normal, but gammaglobulin levels were markedly decreased (IgG 283 mg/dl, IgA 17 mg/dl, IgM 1 mg/dl). Clinical remission of MG was achieved by thymectomy followed by high-dose corticosteroids. Despite monthly intravenous immunoglobulin supplementation, he suffered from repeated respiratory tract infections and candidiasis. Body CT revealed adrenal tumor and pancreatic cancer with liver metastasis, and he died of bacterial pneumonia. Immunological evaluations showed complete lack of CD19+ B cell in the peripheral blood and responses of peripheral blood mononuclear cells to mitogens. Peripheral blood T cells responded to a suboptimal concentration of a recombinant AChR fragment: this pattern of AChR-induced T cell response was typical of MG patients. We failed to detect IgG autoantibodies reactive with B cells in his serum. Patients with Good syndrome represent imbalance of immune responses, leading to both immunodeficiency and autoimmunity.