We herein report a case of chronic hepatitis C where the patient developed severe thrombocytopenia during interferon therapy. The patient was a 61-year-old woman, who received interferon therapy on April 27, 1993 under the diagnosis of C type chronic active hepatitis. After 4 weeks, her platelet count had decreased to 18,000/microliters and intraoral hemorrhage had begun. Although she received 250 mg of methylprednisolone and 20 U of platelet transfusion three times, her platelet count continued to decrease to 4,000/microliters on both May 28, and on June 3, 1993, and so she was transferred to our hospital on June 4. On her second admission to our hospital, although the platelet-associated IgG (PA-IgG) had increased markedly and the megakaryocytes in her bone marrow had decreased, her platelet count had already increased to 37,000/ microliters, and this gradually returned to a normal level accompanied with a decrease of PA-IgG within one month In this case, although we found immunological abnormalities (high level of IgG, positive ANA and positive anti-smooth muscle antibody) prior to interferon treatment, we could not diagnose the patient as having suffered from autoimmune disease, including autoimmune hepatitis, because she did not satisfy the necessary criteria and because she did not have any symptoms suggesting autoimmune disease. We consider that there may be the possibility that interferon induced only an anti-platelet antibodies that caused the high level of PA-IgG and decreased the production level of platelets within the bone marrow.