Thymomas are tumors originating from tyhmic epithelium that should be considered in the differential diagnosis of adult patients with hypogammaglobulinemia. Thymomas are usually detected in patients over 40 with myasthenia gravis or in asymptomatic patients with a mediastinal mass recognized during a routine check-up. Patients with thymoma who were referred to a chest surgery clinic for removal of the tumor should be evaluated in terms of an accompanying immune deficiency (Good's syndrome). Ignorance or a delay in the diagnosis may cause death due to severe infections associated with opportunistic pathogens. It should be noted that hypogammaglobulinemia might be detected many years after the removal of the thymoma and so it would be appropriate to follow such patients regularly in an immunology department.