In a study of 29 cycles of IVF, ET was performed on day 5 after oocyte recovery when embryos had developed to the morula/blastocyst stage. Three preclinical pregnancies and three live births resulted (2 singleton and 1 twin), giving a viable PR per ET of 10%. It is concluded that while day 5 ET may well be important in terms of embryo biopsy for the preimplantation diagnosis of genetic disease, day 2 ET remains preferable for therapeutic IVF. Although these data would not support the introduction of day 5 ET into routine therapeutic IVF, delayed ET should be considered as an alternative approach to preimplantation diagnosis. Indeed, because the latter will generally involve the treatment of normal, fertile couples, it might be predicted that embryo survival rates, and thus the rate of pregnancy after day 5 ET, would be better than those presented here.