The nosocomial transmission of hepatitis C involving three subjects participating in a research project in a Swedish hospital is described. One person contracted acute hepatitis C and another participant had an asymptomatic seroconversion. A temporal relationship between participation in the experiments and seroconversion for hepatitis C, and virology with phylogenetic analyses, suggest that they most probably contracted hepatitis C from a third subject who unknowingly had chronic hepatitis C. Faulty nursing and procedural routines were identified as the probable cause of the transmission. Our findings reinforce the need for medical personnel to strictly adhere to universal precautions worked out to prevent the transmission of blood-borne viruses and underline the importance of a continuous supervision of nursing and procedural routines, even in hospitals with well-educated staff and presumably high standards.