Background. In vitro fertilization, sometimes involving the cryopreservation of human embryos, has become a routine procedure for the treatment of infertility. Even though there are embryos available for transfer in about 85 percent of the treatment cycles, the rate of pregnancy rarely exceeds 25 percent per cycle. We designed this study to investigate two questions: Does this high rate of failure result from inadequate technique, or does it simply reflect the maximal potential of a cohort of aspirated eggs to produce a pregnancy? And to what extent does cryopreservation affect the capacity for implantation of embryos? Methods. The study was conducted among patients enrolled in an egg-donation program. Aspirated eggs from a given cohort were distributed to the donor herself and a few recipients. The recipients were prepared by a standard protocol of hormone replacement and were assigned at random to the transfer of either fresh or frozen and thawed embryos. The donors received only fresh embryos. Result. Forty cycles of donation were studied. In 25 cycles (63 percent) pregnancy was established in the donor, in the recipient (or recipients), or in both. Of the fresh embryos that were transferred to the recipients, 24 percent were successfully implanted, as compared with only 7.7 percent of the frozen and thawed embryos (P<0.01). A pregnancy success rate of 37 percent per recipient cycle was observed in the recipients of fresh embryos, as compared with a rate of only 16 percent in those receiving frozen and thawed embryos (P<0.05). Conclusion. The majority of egg cohorts evidently possess the potential to produce a pregnancy, but cryopreservation of human embryos significantly reduces their capacity for implantation. (N Engl J Med 1990; 323: 1153–6.).