To determine the effect of various freezing protocols on postthaw development and pregnancy rates resulting from transfer of human zygotes. Prospective study. Tertiary care center. Couples undergoing assisted reproductive technology (ART) procedures who wished to have their excess zygotes cryopreserved. We cryopreserved zygotes with one of three protocols. Post- thaw survival and development of the zygotes as well as pregnancy rate after transfer of these zygotes. A 3minute hold time after seeding, followed by a final preplunging temperature of - 180℃ , resulted in a clinical pregnancy rate of 28.6% . In contrast, a 15- minute postseed hold time and a - 30℃ final chamber temperature resulted in a 37.3% clinical pregnancy rate. When we combined the protocols to provide a 15- minute postseed holding time and a - 180° C before plunging into liquid nitrogen, we achieved a 69.6% clinical pregnancy rate. By increasing the postseeding hold time and decreasing the temperature of the freezing chamber before plunging the zygotes into liquid nitrogen, significant improvements can be made in postthaw development and pregnancy rates. To determine the effect of various freezing protocols on postthaw development and pregnancy rates resulting from transfer of human zygotes. Prospective study. Tertiary care center. Couples undergoing assisted reproductive technology (ART) procedures who wished to have their excess zygotes cryopreserved. We cryopreserved zygotes with one of three protocols. Post-thaw survival and development of the zygotes as well as pregnancy rate after transfer of these zygotes. A 3minute hold time after seeding, followed by a final preplunging temperature of - 180 ° C, resulting in a clinical pregnancy rate of 28.6%. In contrast, a 15- minute postseed hold time and a - 30 ° C final chamber temperature resulted in a 37.3% clinical pregnancy rate. When we combined the protocols to provide a 15- minute postseed holding time and a - 180 ° C before plunging into liquid nitrogen, we achieved a 69.6% clinical pregnancy rate. By increasing the postseeding hold time and decreasing the temperature of the freez ing chamber before plunging the zygotes into liquid nitrogen, significant improvements can be made in postthaw development and pregnancy rates.