Late secretory endometrium synthesizes and secretes progestagen-associated endometrial protein (PEP), which is measurable in the serum of cycling women, and has been shown to increase in concentration during the luteal phase of the normal menstrual cycle. The purpose of this study was to determine the utility of serum PEP as a predictor of normal or inadequate luteal phase endometrial differentiation. One-hundred-and-twenty-five endometrial biopsies were taken within 4 days of a subsequent menstrual period, during the course of evaluation of infertility or recurrent abortion. Twenty-one of these biopsies demonstrated glandular/stromal asynchrony. Thirteen patients exhibited 'out-of-phase' endometrial biopsies and 91 patients had 'in-phase' biopsies. Mean PEP values significantly increased from the mid-luteal phase to the late luteal phase. Mean PEP values for patients with in-phase biopsies (43.92 units/ml) were not significantly different from those with out-of-phase biopsies (25.24 units/ml, P = 0.0942). PEP values for patients with asynchronous biopsies were intermediate. Use of clomiphene citrate did not affect these results.