An investigation was carried out to find the source of infection in a patient who developed hepatitis C virus (HCV) infection after cardiothoracic surgery, and to determine whether other patients had become infected. Virological tests on specimens from the patient and potential sources (blood donors and members of the surgical team) showed that the patient and the surgeon who acted as first assistant were infected with HCV of the same genotype: 4a. No other source of infection was identified. Ninety-one per cent (277) of the 304 other exposed patients available for follow up were tested--none had antibody to HCV. It was concluded that hepatitis C may be transmitted from surgeon to patient during exposure prone procedures, and that the transmission rate in this incident was 0.36% (1/278; 95% confidence interval 0.0061%-1.98%).