We report the case of a 65-year-old man with myasthenia gravis, who developed recurrent opportunistic infections following thymectomy and immunosuppressive therapy. Subsequent evaluation including immunological studies, flow cytometry, and bone marrow studies confirmed the diagnosis of Good's syndrome. The patient was successfully treated with intravenous immunoglobulin (IVIG) and has remained stable with a monthly IVIG regimen. Good's syndrome should be strongly suspected when patients with myasthenia gravis develop recurrent opportunistic infections, especially after discontinuation of immunosuppressive therapy. Any delay in diagnosis can be life-threatening in such patients. Serum immunoglobulin levels and flow cytometry should be considered part of the initial diagnostic evaluation in patients with myasthenia gravis and an anterior mediastinal mass prior to the initiation of immunosuppressive therapy.