AbstractObjective: Placental protein 14 (PP14) is known to be one of the endometrial proteins that reflect endometrial functioning throughout the menstrual cycle. In this study, we examined PP14 as a marker for human endometrial receptivity in order to predict the outcome of in vitro fertilization and the embryo‐transfer (IVF‐ET) cycle.Patients and Methods: The subjects were 72 women who had 96 IVF‐ET cycles and who were examined at Tokyo Medical University Hospital during the period of January 1998 to June 1998 because of mechanical or unexplained infertility for a duration of at least 2 years. Serum samples were collected from all patients during treatment cycles, and serum PPI4 concentrations were measured by a newly established enzyme‐linked immunosorbent assay (ELISA).Results: In the pregnant group, serum PP14 concentrations were markedly increased after ET, and a significant difference between the pregnant group and the nonpregnant group was observed 8 days following ET (p < 0.01). PP14 concentrations were higher in patients with endometria that exhibited homogenous patterns and that were more than 7 mm thicker than in other patients, as determined by ultrasound on the day of oocyte collection (p < 0.005). The pregnancy rates of patients with homogeneous patterns were lower than those of patients showing a trilaminar pattern. No pregnancies were observed when serum PP14 concentrations were greater than 6.85 U/l on the day of oocyte collection.Conclusion: PP14 might be a useful marker for human endometrial receptivity to predict the outcome of IVF‐ET cycles.