['To examine the relation between blood found on the transfer catheter after ET and the rates of embryo implantation and clinical pregnancy with the use of IVF-ET.', 'Retrospective cohort study.', 'A tertiary care center for assisted reproductive technology.', 'Three hundred seven couples who underwent 354 ETs between January 1, 1994, and June 30, 1996.', 'A semiquantitative system for recording the amount of blood found inside and outside the transfer catheter after ET.', 'Embryo implantation rate and clinical pregnancy rate (PR).', 'Blood found outside the transfer catheter after ET was associated with decreased rates of embryo implantation and clinical pregnancy. In contrast, blood located inside the transfer catheter after ET, the type of catheter used for ET, the number of transfer attempts, and the time required for ET did not significantly affect either the embryo implantation rate or the clinical PR.'] Is blood on the embryo transfer catheter associated with decreased rates of embryo implantation and clinical pregnancy with the use of in vitro fertilization-embryo transfer?