Good syndrome is a rare condition characterized by the presence of thymoma in combination with adult-onset hypogammaglobulinemia. Immunological features of Good syndrome include various immunodeficiencies accompanied with hypogammaglobulinemia. In patients with thymoma, paraneoplastic syndromes including hypogammaglobulinemia worsen the prognosis. We herein describe a patient with advanced-stage type A thymoma who was effectively treated with chemotherapy and exhibited a parallel decrease in the serum level of soluble interleukin-2 receptor (sIL-2R), which depends on cellular immunity. The present case suggests the efficacy of sIL-2R as a potential prognostic biomarker in a subset of patients with Good syndrome.