We report the case of a 40-year-old woman with chronic viral hepatitis C and latent idiopathic thrombocytopenic purpura who developed severe thrombocytopenia during alpha interferon therapy. Platelet-associated IgG titers were elevated, and platelet antibodies were detected in the serum. Intravenous administration of high-dose polyvalent immunoglobulins was ineffective, but a normal platelet count was obtained after corticosteroid therapy. A history of idiopathic thrombocytopenic purpura could be considered a contraindication for alpha-interferon therapy in patients with chronic viral hepatitis.