We report the case of a 41-yr-old male with chronic hepatitis C who developed severe thrombocytopenia on interferon alfa-n3 therapy. The patient was enrolled in a multi-center trial of interferon alfa-n3 in the treatment of chronic hepatitis C. In the 10th wk of therapy, he presented with gingival bleeding and a petechial rash. Complete blood count demonstrated WBC 5,800/mm3. Hgb 16.3 g/dl, Hct 45.5%, and platelet count 6,000/mm3. Interferon was withdrawn, and he was admitted to intensive care and was treated with platelet transfusion, intravenous steroids, and intravenous immunoglobulin. His platelet counts returned to 157,000/mm3 in 7 days and remained normal on a tapering dose of prednisone. Bone marrow biopsy/aspirate demonstrated numerous megakaryocytes. Stored plasma was negative for development of specific high-titer anti-platelet antibody. We suggest that the clinical findings and response to therapy are consistent with interferon-induced idiopathic thrombocytopenic purpura.