Background. Good's syndrome is a rare immunodeficiency condition that develops in association with thymoma. It has been reported to occur in 0.2-0.3% of thymoma patients in Japan. Case. A 52-year-old male underwent an extended thymo-thymectomy at our institution and received a pathological diagnosis of invasive thymoma. After 11 years, follow-up computed tomography results showed signs of pleural dissemination, which was treated with chemotherapy. The disseminated tumor was resected 15 years after the initial diagnosis, and the pathological diagnosis was dissemination of invasive thymoma. There were no postoperative complications. At four months after the second surgery, the patient developed diarrhea, accompanied by weight loss of 5 kg/month. The cause was unclear. The diarrhea remained intractable despite treatment with probiotics and antidiarrheal drugs. During an examination to investigate the patient's condition, early esophageal cancer was incidentally found, for which an endoscopic submucosal dissection procedure was performed. During hospitalization, the patient developed aspiration pneumonia, which led to sepsis. Test results indicated hypogammaglobulinemia, along with a decrease in B cells and CD4-positive T cells, and the patient was diagnosed with Good's syndrome. The patient was diagnosed with Good's syndrome 16 years after the initial diagnosis of thymoma. Immunoglobulin therapy was introduced, but the patient developed sepsis due to pneumonia and died five months later. Conclusion. Chronic diarrhea is known to occur in approximately half of patients with Good's syndrome, and should be kept in mind to avoid delaying the diagnosis.