During a one-year period 636 excess embryos obtained after in-vitro fertilization and gamete intra-Fallopian transfer combined with in-vitro fertilization were cryopreserved using two different protocols. For early stage embryos including the pronucleate stage, 1,2-propanediol was used as cryoprotectant (procedure A, adapted from Renard) and for later stage embryos dimethylsulphoxide was used in protocol B, adapted from Trounson and Mohr. After thawing 288 embryos, half of them were of sufficient quality to be replaced. After cryopreservation, procedure A gave the best survival in embryos having less than or equal to 2 blastomeres; for later stage embryos best survival was obtained using the dimethylsulphoxide protocol. Survival after cryopreservation was also clearly related to the quality of the embryos prior to freezing. Embryos were replaced during endocrinologically monitored natural cycles and were transferred in synchrony between endometrial and embryonic age. After replacement of 126 embryos in 110 patients, 20 pregnancies occurred. So far six healthy children have been born, two patients aborted and 12 pregnancies are ongoing. In this series no statistical difference was observed between the implantation rate of embryos cryopreserved by procedure A or B. Six pregnancies occurred in patients from the oocyte and embryo donation programme. An adequate cryopreservation programme circumvents the difficult problem of synchronizing the ovarian cycles of donor and acceptor patients.