The timing of ET was evaluated by transferring four embryos at 44 to 48 hours, 68 to 72 hours, or equally dividing and sequentially transferring at 44 to 48 and 68 to 72 hours after insemination. Fifty-one patients were randomly allocated to one of the above protocols. The mean number of blastomeres of embryos transferred at 68 to 72 hours after insemination was significantly (P < 0.0001) higher than those transferred at 44 to 48 hours. The number of embryos with good morphology was similar in all study groups. The pregnancy rate was similar in the three protocol groups, 53% at 44 to 48 hours, 59% at 64 to 72 hours, and 47% by sequential transfer. There probably is no advantage in delaying the transfer or dividing the embryos into two sequential transfers. It seems that sequential transfer is not associated with any harmful effect.