In 14 cycling women participating in an in-vitro fertilization (IVF) donation programme, we examined the timing of the 'nidation window' using as a stage-specific 'marker' the presence of fully developed pinopodes on the apical surface of the luminal uterine epithelium. Each woman received exogenous oestradiol from the second day of their cycle and progesterone starting on day 8 or day 15 of the oestrogenic treatment. The women underwent two biopsies during the same artificial cycle, on either days 6 and 9 or days 8 and 10 of the progesterone treatment. All patients to whom oestradiol was administered for 7 days prior to progesterone administration (n = 9), and two of the five treated with oestradiol for 14 days prior to the addition of progesterone, showed uterine pinopodes in either one or both biopsies. When present on a given day, pinopodes were at the same stage, developing, fully developed or regressing, showing that their total lifespan did not exceed 48 h. Fully developed pinopodes existed for 1 day only which may correspond to the short period of optimal endometrial receptivity observed in animal models. The timing of the presence of fully developed pinopodes varied from patient to patient, but these individual differences were not correlated with progesterone and oestradiol plasma concentrations. The brief duration of the nidation window and the observed individual variations in its timing suggest that the detection of uterine pinopodes could be a valuable tool for the prediction, on an individual basis, of the optimal date for successful egg replacement in IVF patients.