The success rate of human embryo cryopreservatlon depends on technical and embryonic parameters. First of all, the cryoprotectant can affect embryo survival as we found by comparing two freeze-thaw procedures using propanediol (PROH) (1.5 mol) alone or with sucrose (0.1 mol). Embryo survival was significantly enhanced with sucrose (62 versus 32%). Embryo quality is another major parameter involved in the success of freezing; the rates of positive survival were found to be 67% for morphologically normal embryos versus 49% for embryos with fragments (P < 0.001). The efficiency of embryo cryopreservatlon in an IVF programme could be estimated in 1986: a woman with extra embryos, stored after transfer of 3–4 fresh embryos (16% of all cydes), can expect a 22% pregnancy rate per transfer of fresh embryos and a 32% pregnancy rate per collection after transfer of the stored eggs. A comparative study of the cryopreservability of immature or mature oocytes was performed in humans. Human oocytes have a low survival rate (36%) whatever the cryopreservation protocol or the initial maturation stage. Immature human oocytes could survive freezing and thawing, mature and be fertilized in vitro, but with a very low efficiency.