The hypothesis was tested that the medium used to culture embryos affects the concentration of human chorionic gonadotrophin (HCG) early in pregnancy. The value of these concentrations in predicting successful outcome was also assessed for each medium studied. Patients undergoing IVF between January 1998 and December 2004 and having a day 3 embryo transfer were stratified into one of four groups according to the medium in which their embryos were cultured (P1, IVF500, G1.2, and G1.3). Using receiver operating characteristic (ROC) curve analysis, cut-off values for serum HCG concentrations on day 15 after embryo transfer were calculated for optimal discrimination between cycles resulting in implantation failure and success for each medium. Cut-off points were chosen to maximize sensitivity and specificity. For viable singleton pregnancies, mean HCG concentrations were greater for G1.3 and lower for IVF500 compared with the other media. Discriminatory HCG cut-off concentrations for predicting implantation success were lowest for IVF500, intermediate for P1 and G1.2 and highest for G1.3. The data support the hypothesis that the medium used to culture embryos significantly affects the concentrations of HCG early in pregnancy. Furthermore, when using HCG cut-off concentrations to assess pregnancy outcome, medium type should be taken into consideration.