A 64‐year‐old Japanese woman with thymoma has been suffering from diarrhea and increased alkaline phosphatase levels without jaundice. Her serum immunoglobulin levels of IgM and IgG were less than half of the normal levels, with an increase in CD8 (suppressor/cytotoxic) T cell percentage and a decrease in CD4 (helper) T cell percentage, resulting in a lower CD4/CD8 ratio of 0.31. These immunological features are in accordance with those of hypogammaglobulinemia complicating thymoma. Cholangiography and a liver biopsy specimen disclosed the presence of primary sclerosing cholangitis (PSC). PSC has been recognized in various immunodeficiency syndromes and this case shows that thymoma complicated by hypogammaglobulinemia is associated with