To determine the effect of cryopreservation on embryo quality and the pregnancy potential of embryos, donated oocytes from the same donor (n = 24) were randomly allocated, with subsequent transfer to two or more different ovum recipients resulting in at least one fresh and one frozen embryo transfer cycle from the same cohort of oocytes. Endometrial receptivity was controlled in all ovum recipients, and male factor patients were excluded. The number of embryos transferred, mean embryo grade transferred, number of high quality embryos (grade < or = 2.5, grade 1 being best) transferred and embryo implantation and live birth rates are reported. Significantly more embryos (4.4 +/- 1.2 versus 3.3 +/- 1.2, P < 0.00003) of higher quality (1.9 +/- 0.5 versus 2.1 +/- 0.5, P < 0.013) and of a more advanced cell stage (3.0 +/- 0.6 versus 2.6 +/- 0.7, P < 0.019) were transferred fresh than after cryopreservation respectively. Implantation rates/embryo [19/151 (12.6%) and 9/111 (8.1%)] and live birth rates/transfer [11/42 (26.2%) and 6/45 (13.3%)], from fresh and frozen transfers respectively, were not significantly different despite the larger number of high quality embryos transferred fresh. Embryo cryopreservation adversely affects embryo quality, but does not have detrimental effects on the implantation or pregnancy potential of high quality embryos. Because of the loss of embryos during freeze-thawing during frozen embryo cycles, every effort should be made to attempt a fresh transfer.