The IgM and IgG antibody response to various hepatitis C virus (HCV) antigens was studied in 8 patients who acquired posttransfusion HCV infection. IgM anti-HCV was detectable in only 4 of these patients, coincident with (1 patient) or later than (3 patients) the IgG anti-HCV response. Seven patients had initially decreasing IgG anti-HCV titres, indicating passive transfer of antibodies from donor to recipient. All 8 patients showed active IgG seroconversion, as demonstrated by increasing IgG anti-HCV titres, on average, 75 days after infection. Five years after infection, all patients were still reactive for IgG anti-HCV antibodies and 7 were positive for HCV RNA by the polymerase chain reaction (PCR). Two of these PCR positive patients were also reactive for IgM anti-HCV. It is concluded that the serology of HCV infection does not follow the classical pattern of IgM response preceding detection of IgG. The IgM response may be absent, late, or persistent after HCV infection. The serological diagnosis of recent HCV infection should be based on the polymerase chain reaction or rising IgG titres in at least 2 sequential patient blood samples.