To assess the incidence and source of community-acquired hepatitis C virus (HCV) infection among subjects at low risk for blood-borne diseases, we prospectively studied a cohort of 16,515 repeat blood donors over a mean follow-up time of 36 months. Second- and third- generation methods were used for hepatitis C virus antibody (anti-HCV) testing. HCV RNA was determined in the serum of anti-HCV-positive donors by reverse-transcription polymerase chain reaction. Liver biopsy was performed in the viremic subjects. Risk factors for HCV infection were identified by a psychosocial questionnaire in the whole cohort. During follow-up, 5 donors became infected with HCV. The incidence was 1 per 10,000 person-years (95% confidence interval, 0.3-2.4 per 10,000). During the 6 months before seroconversion, four subjects (80%) underwent medical or surgical percutaneous procedures, compared with 26.5% in the entire donor cohort (difference between frequencies, 53.5%; CI: 18.9-89.1). One seroconverting donor had sexual intercourse with an infected subject. Only 1 infected donor developed clinically evident acute hepatitis. HCV RNA remained detectable in 4 of 5 subjects for 8 to 36 months after seroconversion, and liver biopsy showed chronic hepatitis in all cases. Thus, new cases of hepatitis C occur among individuals without a history of known risk factors, some of which may be caused by nosocomial exposure.