The study was a nonconcurrent prospective, seroepidemiologic study. To estimate the incidence of HCV infection, six-month postoperative serum samples were tested for antibody to HCV. Preoperative samples corresponding to positive postoperative sera were also tested. The donors of a patient who received blood screened for HCV, i.e., a donation since May, 1990, and seroconverted in the six months since transfusion were located and retested for antibodies to HCV. The donors were administered a questionnaire designed to ascertain risk factors for HCV infection. A sample of donors who were HCV positive at the time of donation were given the same questionnaire. Comparison of these three groups of donors (i.e., seropositives, donors tested negative but implicated in seroconversion, seronegatives) allowed an investigation of factors associated with false negative screening results. Analytical methods included standard methods for contingency tables and both conditional and unconditional logistic regression.