Although mild thrombocytopenia is a common adverse effect of interferon therapy, severe life-threatening thrombocytopenia is extremely rare. Here, we report a case of chronic hepatitis C patient that developed severe thrombocytopenia during alpha-interferon therapy, possibly due to an autoimmune mechanism. A 24-year-old female presented chronic hepatitis C in May, 1998. Based on the clinicopathological findings including a liver biopsy, administration of alpha-interferon was begun. In the fourth week of therapy, she experienced mild dyspnea and general fatigue. Complete blood count demonstrated thrombocytopenia (48,000/microL). Despite the immediate withdrawal of interferon, her platelet count further decreased to 1,100/microL. Bone marrow aspirate and elevated platelet-associated IgG antibodies were suggestive of immune thrombocytopenia. She was treated with intravenous and oral administration of steroids. Her platelet count returned to normal level 5 days later. Response to steroid treatment was consistent with the diagnosis of alpha-interferon-induced immune thrombocytopenia in this patient.