To assess the value of routine screening of nuclear status of day- 2 four-cell preembryos for single embryo transfer (SET) in predicting implantation. Design: Retrospective analysis. Setting: Private IVF unit. Patient(s): A total of 1,985 fresh embryo transfers on day 2 or day 3 were performed from January 2002 to November 2004. In 1,295 (65.2% ) of these transfers, SET was performed. All day- 2 four-cell preembryos transferred in SET cycles (n = 861) were analyzed retrospectively for utcome in terms of implantation rate and its relation to the number of visible mononucleate blastomeres (MNBs), the degree of fragmentation, and equality of blastomeres. Intervention(s): Light microscopic evaluation of preembryos before transfer on day 2. Main Outcome Measure(s): Implantation rate. Result(s): The number of MNBs was found to be related to implantation, whereas blastomere equality and rate of embryo fragmentation were not. The implantation rate was statistically significantly higher in cycles where a four-cell preembryo with four MNBs was transferred than after transfer of a four-cell preembryo with zero to three MNBs (42% versus 22% ). In a logistic regression analysis, nucleation of all blastomeres was the only morphologic parameter that was associated with the implantation rate. Conclusion(s): Evaluation of nuclear status of four-cell preembryos is important in predicting implantation potential. Visualization of four MNBs in a four-cell preembryo predicted a statistically significant higher implantation rate than in cases where not all four blastomeres were mononucleate. To assess the value of routine screening of nuclear status of day- 2 four-cell preembryos for single embryo transfer (SET) in predicting implantation. Design: Retrospective analysis. Setting: Private IVF unit. Patient (s): A total of 1,985 fresh embryo transfers on day 2 or day 3 were performed from January 2002 to November 2004. In 1,295 (65.2%) of these transfers, SET was performed. All day- 2 four-cell preembryos transferred in SET cycles (n = 861) were analyzed retrospectively for utcome in terms of implantation rate and its relation to the number of visible mononucleate blastomeres (MNBs), the degree of fragmentation, and equality of blastomeres. Intervention (s): Light microscopic evaluation of preembryos before transfer on day 2. Main Outcome Measure (s): Implantation rate. Result (s): The number of MNBs was found to be related to implantation, but blastomere equality and rate of embryo fragmentation were not. The implantation rate was statistically significantly higher in cycl es where a four-cell preembryo with four MNBs was transferred than after transfer of a four-cell preembryo with zero to three MNBs (42% versus 22%). In a logistic regression analysis, nucleation of all blastomeres was the only morphologic parameter that Conclusion (s): Evaluation of nuclear status of four-cell preembryos is important in predicting implantation potential. Visualization of four MNBs in a four-cell preembryo predicted a significant significant higher rate than in cases where not all four blastomeres were mononucleate.