A 63-year-old man was admitted to our hospital for treatment of hypogammaglobulinemia with thymoma (Good's syndrome). Tests for immunological function showed an abnormality in humoral immunity with decreases in the proportion of cells bearing B-cell markers in the peripheral blood and bone marrow. The patient was found to have Campylobacter fetus sepsis caused by the hypogammaglobulinemia due to humoral immunodeficiency, and he was given gamma-globulin supplement. Thymectomy was performed due to enlargement of the thymoma after 4 years of follow-up and the pathological diagnosis was thymoma of the non-encapsulated, epithelial spindle cell type. Although there was no recurrence of thymoma, the hypogammaglobulinemia remained unchanged and the patient continued to suffer from repeated infections. Thus, we describe the case of a patient with Good's syndrome associated with independent humoral immunodeficiency in whom the effect of thymectomy for hypogammaglobulinemia was negative. In this respect, thymectomy was only beneficial when the thymoma proliferated and seemed to be more threatening than the hypogammaglobulinemia for the patient.