Post-transfusion hepatitis C incidence was studied in a series of patients with bone marrow allograft. The risk of HCV seroconversion was evaluated according to the date of grafting and the screening tests carried out in blood donors at this time. Anti-HCV antibodies were screened using Elisa tests of 2d generation and confirmed by Riba tests of 2d generation. Results were analysed. Out of 181 allografted patients from January 1987 to December 1991, 120 patients found anti-HCV negative prior to grafting, with at least six month post-transfusion follow-up were considered as evaluable in terms of HCV seroconversion. All these patients had received leucodepleted blood products and the most of them platelet unit concentrates. Prior to implementation of screening tests for non-A, non-B hepatitis, 14% of patients had seroconverted (0.44% per transfused product); after introduction of the screening for indirect markers (ALAT) and for antibodies directed against the antigen of hepatitis B virus core (anti-HBc), the seroconversion incidence was 4% (0.26% per product). At the present time, since the implementation of anti-HCV screening tests, the risk has reached 1.6% (0.03% per transfused product). 6 patients out of 7 having seroconverted have been developing chronic hepatitis.