OBJECTIVE: To determine whether premature progesterone elevation affects the timing of hyperechogenic transformation of the endometrium during the early luteal phase of controlled ovarian hyperstimulation (COH) cycles. DESIGN: Prospective analysis. SETTING: Assisted Reproduction Unit, Hopital Antoine Beclere, Clamart, France. PATIENT(S): Fifty-nine women undergoing 59 IVF-ET cycles. INTERVENTION(S): Patients underwent COH with a GnRH agonist and hMG. Endometrial echogenicity was assessed on the days of hCG administration, oocyte retrieval, and ET. Results are expressed as the extent of submyometrial hyperechogenic area in relation to the total endometrial surface as determined by a computer-assisted analysis system. Patients were sorted according to whether their plasma progesterone level exceeded 0.9 ng/mL (n = 26) or not (n = 33) on the day of hCG administration. MAIN OUTCOME MEASURE(S): Endometrial echogenicity. RESULT(S): On the day of hCG administration, the degree of endometrial echogenicity was similar in both groups (41% vs. 40%), but after hCG administration, it increased significantly faster in the high progesterone group than in the low progesterone group (70% vs. 63% at oocyte retrieval and 90% vs. 79% at ET, respectively). CONCLUSION(S): End-follicular phase elevation in plasma progesterone (>0.9 ng/mL on the day of hCG administration) was associated with a faster increase in endometrial echogenicity during the early luteal phase of COH cycles. This observation is consistent with the hypothesis that premature progesterone elevation hastens the secretory transformation of the endometrium.