We have analysed anti-HBc and anti-HCV antibodies in serum samples from 799 donors which had their blood or derivates transfused to 111 recipients. Anti-HBc and anti-HCV were reactive in respectively 9 and 2.1% of the donors tested. We have observed that among the 111 recipients, 44 had received at least one positive anti-HBc unit and 67 had been transfused only with negative anti-HBc, units. The risk of developing hepatitis C virus was 4.5 times higher for the recipients who received at least one positive anti-HBc unit. If the test for anti-HBc had been made for the blood donors in the serological screening, about 56% of the HCV cases in the recipients could have been avoided. The population of recipients who received at least one reacting unit of anti-HCV, presented a risk 29 times higher of developing this hepatitis, as compared to the transfused recipients with all anti-HCV negative units. Testing blood from donors for anti-HCV would avoid 79% of the post-transfusional HCV cases. Brazilian candidates to blood donors seem to be carriers either simultaneously or sequentially to hepatitis virus B and C, since 44.4% of the positive anti-HCV were also positive for anti-HBc. Testing for anti-HBc and anti-HCV in blood screening must be indicated in order to prevent post-transfusional hepatitis transmission in our community.