['To evaluate the sex ratio of offspring born after blastocyst transfers.', 'Retrospective data analysis.', 'A large assisted reproductive technology center.', 'We included 1,284 offspring from 937 deliveries during the period August 2003-August 2005.', 'Tabulation and statistical analysis of all births resulting from fresh IVF cycles. The sex of resulting offspring was compared in both day 3 and blastocyst transfers for all births and for singleton deliveries. In addition, the sex of children conceived with the use of autologous oocytes and donor oocytes was evaluated.', 'Sex ratio of offspring born following embryo transfers (ETs) after day 3 of culture and sequential blastocycst culture.', 'The overall sex ratio was significantly shifted toward males when blastocyst transfers were performed. Blastocyst transfers with only the use of autologous oocytes resulted again in a significantly higher proportion of male offspring. An even greater proportional difference was encountered in singleton offspring from donor oocytes. However, significance was not reached because of the limited number of offspring in the subgroup.'] Is blastocyst embryo transfer associated with a sex-ratio imbalance in favor of male offspring?