The polymerase chain reaction was used to detect hepatitis C virus infection in patients who had previously been reported to have developed non-A, non-B hepatitis after intravenous immunoglobulin infusion. Of the 33 patients with intravenous immunoglobulin associated non-A, non-B hepatitis studied, HCV RNA could be detected in 15 out of 17 patients (88%) who were HCV RNA negative prior to the development of non-A, non-B hepatitis after implicated intravenous immunoglobulin batches. Similarly, eight out of nine patients (89%) in whom no sample was available for polymerase chain reaction testing prior to intravenous immunoglobulin therapy, had detectable HCV RNA after intravenous immunoglobulin therapy with intravenous immunoglobulin batches implicated in non-A, non-B hepatitis transmission. Two of the three intravenous immunoglobulin preparations implicated in non-A, non-B hepatitis transmissions that were available for polymerase chain reaction testing also had detectable HCV RNA, confirming that hepatitis C virus is the implicated virus in intravenous immunoglobulin-associated non-A, non-B hepatitis.