['To analyze the effects of patient age and treatment cycle number on the occurrence of blastocyst transfer and subsequent implantation.', 'Prospective study.', 'Department of endocrinology and reproduction.', 'All 1,099 women had day-5 transfers after IVF or intracytoplasmic sperm injection treatment.', 'All patients were checked for embryo development in vitro in consecutive day-5 transfer cycles. Two blastocysts or three lesser-developed embryos were transferred.', 'Blastocyst formation rate or clinical pregnancy/implantation rate.', 'Of 929 patients in the first cycle, 545 (59%) had at least one blastocyst available for ET. Among 151 patients with a blastocyst in cycle 1, 77 developed one or more blastocysts in cycle 2 (51%). Fifty of 143 patients without a blastocyst in cycle 1 had at least one blastocyst in cycle 2 (35%). After subdivision of all day-5 ETs according to the first four cycles, the following implantation rates per embryo were found for ET with one or more blastocysts: cycle 1 (n = 545), 23%; cycle 2 (n = 264), 23%; cycle 3 (n = 110), 14%; and cycle 4 (n = 27), 12%, and with noncavitating embryos, respectively: (n = 384) 6%, (n = 193) 6%, (n = 94) 2%, and (n = 35) 3%. The negative correlation of the age of the woman on blastulation depended primarily on the number of oocytes retrieved.'] Does blastocyst transfer in day-5 embryo transfer depend primarily on the number of oocytes retrieved and not on age?