An episode of acute hepatitis in a patient with hemophilia during immunoadsorption therapy initially was misinterpreted as a reactivated hepatitis C virus (HCV) infection, but ultimately was shown to be an exogenous reinfection during cohort treatment with another HCV-positive patient. This incident illustrates that policies for the prevention of nosocomial transmission of blood-borne pathogens, especially in cohort treatment units, may need to be reassessed.