To assess the value of reinseminating human oocytes, the results of 1,662 embryo transfers were analyzed. In 1,460 transfers embryos arose from oocytes that had fertilized and cleaved after initial insemination. The pregnancy rate was significantly higher than in transfers of embryos (n = 76) resulting solely from reinseminated oocytes (27% versus 3%). Adding reinseminated embryos to those fertilizing on initial insemination at transfer failed to raise the pregnancy rate. Only 2 of 158 (1.3%) reinseminated embryos implanted compared with 540 of 4,181 (12.9%) fertilized and cleaved on initial insemination (P less than 0.001). Reinseminated embryos do occasionally produce viable pregnancies. It is therefore worth considering replacement of these embryos if initial fertilization has entirely failed. Patients should be counseled as to the low chance of implantation with these embryos.