Objective:To identify the source of an outbreak of acute hepatitis C virus (HCV) infection among 3 patients occurring within 8 weeks of hospitalization in the same ward of a Florida hospital during November 1998.Design:A retrospective cohort study was conducted among 41 patients hospitalized between November 11 and 19, 1998. Patients' blood was tested for antibodies to HCV, and HCV RNA-positive samples were genotyped and sequenced.Results:Of the 41 patients, 24 (59%) participated in the study. HCV genotype 1b infections were found in 5 patients. Three of 4 patients who received saline flushes from a multidose saline vial on November 16 had acute HCV infection, whereas none of the 9 patients who did not receive saline flushes had HCV infection (P= .01). No other significant exposures were identified. The HCV sequence was available for 1 case of acute HCV and differed by a single nucleotide (0.3%) from that of the indeterminate case.Conclusion:This outbreak of HCV probably occurred when a multidose saline vial was contaminated with blood from an HCV-infected patient. Hospitals should emphasize adherence to standard procedures to prevent blood-borne infections. In addition, the use of single-dose vials or prefilled saline syringes might further reduce the risk for nosocomial transmission of blood-borne pathogens.