The transmission of non-A, non-B hepatitis (NANB)/hepatitis C virus (HCV) was studied in patients undergoing open-heart surgery and related to the reception of blood products and hospitalization and surgery per se. Posttransfusion hepatitis NANB was noticed in 17/390 (4.4%) patients receiving heterologous blood and 8/16 tested became positive for anti-HCV (Ortho HCV ELISA), all within 5 months after onset of hepatitis. Among patients with normal ALAT before surgery and during follow-up, who had received heterologous blood, 1/50 seroconverted after 6 months. This patient probably had a subclinical HCV infection, or possibly a temporary non-specific anti-HCV reactivity with a maximum optical density/cut-off ratio (OD/CO) of 1.2, whereas all posttransfusion hepatitis C cases had OD/CO ratios greater than 4. No hepatitis occurred among the 92 non-transfused patients and no seroconversion was found in any of 62 non-transfused patients tested 6 months after the operation. It was concluded that (1) hospitalization and surgery per se does not seem to offer a risk of hepatitis NANB/C, and (2) seroconversion to for HCV occurs in only 50% of Swedish patients with acute posttransfusion NANB hepatitis.