Hepatitis C virus (HCV) infection has been associated with the production of autoantibodies and the development of several autoimmune disorders. Immune thrombocytopenic purpura (ITP) is an immune-mediated syndrome of unknown etiology characterized by the presence of autoantibodies against platelet membrane proteins. Retrospective chart review. Seven patients with chronic HCV infection (five with cirrhosis and two with chronic active hepatitis) developed thrombocytopenia, out of proportion to their liver disease, and were diagnosed with ITP based on the presence of anti-platelet antibodies and their response to treatment. The number of patients with ITP which occurred in a population of 3440 HCV patients seen over this time interval is much greater than would be expected by chance (p < 0.00001). Six patients required treatment and four required hospitalization. Four of the six responded to corticosteroids alone. Both of the patients who failed to respond to corticosteroids responded to cyclophosphamide. No mortality occurred from complications of thrombocytopenia. ITP occurs more commonly in patients with chronic HCV infection than would be expected by chance. This should be considered in patients with liver disease and unexplained thrombocytopenia, as well as in patients with newly diagnosed ITP. Evaluation of antiplatelet antibodies, using an antigen-specific assay, was useful in supporting this diagnosis. Therapy with either corticosteroids or cyclophosphamide was successful in the six patients who required treatment.