A 53-year-old woman who had been diagnosed with rheumatoid arthritis was found to have thrombocytopenia, splenomegaly, and gastric varices. She was diagnosed as having idiopathic portal hypertension on the basis of liver biopsy and angiography. Treatment with prednisolone was not sufficiently effective for thrombocytopenia. After transabdominal devascularization with splenectomy, thrombocytopenia subsided and gastric varices disappeared. In this case, the autoimmune mechanism as well as hypersplenism was suspected of being involved in the mechanism of thrombocytopenia.