We describe the transmission of hepatitis C virus (HCV) to two patients from a thoracic surgeon who was unaware of his hepatitis C infection. By partial sequencing of the non-structural 5B gene and phylogenetic analysis, the viruses from both patients were found to be closely related to genotype 1a strain from the surgeon. Two further hepatitis C cases were found in relation to the thoracic clinic. Their HCV sequences were related to each other but were of genotype 2b and the source of infection was never revealed. To elucidate the magnitude of the problem, we conducted a prospective study for a period of 17 months in which patients who were about to undergo thoracic surgery were asked to participate. Blood samples were drawn prior to surgery and at least four months later. The postoperative samples were then screened for anti-HCV and, if positive, the initial sample was also analysed. The only two patients (0.4%) identified were confirmed anti-HCV positive before surgery, and none out of 456 evaluable cases seroconverted to anti-HCV during the observation period. Despite the retrospectively identified cases, nosocomial hepatitis C is rare in our thoracic unit. The study points out the risk of transmission of hepatitis C from infected personnel and reiterates the need for universal precautions. © 2008 The Hospital Infection Society.