We reported a case of recurrence of localized thymoma accompanied with hypogammaglobulinemia (Good's syndrome) 20 years after surgery. A 74-year-old man was admitted to this hospital because of mediastinal tumor and chronic pulmonary infection. He had been thymectomised at the age of 55 because of spindle cell thymoma. After that, he had been asymptomatic until January 1997, when he began to have a recurrent productive cough, and low-grade fever. Laboratory findings revealed hypogammaglobulinemia. Percutaneous needle biopsy of the mediastinal tumor revealed spindle cell thymoma. Therefore, hypogammaglobulinemia with thymoma (Good's syndrome) accompanied with a chronic lower respiratory tract infection was diagnosed. Immunologic studies revealed a marked decrease of CD 20 positive cells and decreased lymphocyte activation under the stimuli of phytohemagglutinin and concanavalin A. The thymoma was resected in Dec 1997, but the serum immunoglobulin showed no increase at al.