OBJECTIVE To determine if either too little or too much of a rise in serum progesterone (P) on the day after human chorionic gonadotropin (hCG) injection has any negative impact on pregnancy outcome following in vitro fertilization-embryo transfer (IVF-ET). METHODS Retrospective review. Three progesterone groups established--low, normal and high. RESULTS There was a significantly higher clinical pregnancy rate in the normal P group vs low or high. There were no significant differences in ongoing delivered pregnancy rates but a trend for higher implantation rates in the normal P group. CONCLUSIONS These data are consistent with the hypothesis that either too little or too much P can adversely effect implantation. However, the differences are not of sufficient magnitude to warrant a clinical intervention, e.g., deferring fresh transfer and freezing the embryos for future transfer.