In this study we have examined the human endometrial stromal cell population in well-timed biopsies during the peri-implantation period, using traditional stereological techniques. This paper reports data obtained from 16 women of known fertility who underwent endometrial biopsies at known times after the luteinizing hormone (LH) surge (four each at LH + 2, LH + 4, LH + 6 and LH + 8). The average stromal cell nuclear diameter increased in size throughout the period of study (P less than 0.01), with a significant decrease (P less than 0.05) in the nuclear profile axial ratio. This suggests that the nuclei were increasing in size and becoming more rounded. There was a dramatic increase (P less than 0.01) in the rounded. There was a dramatic increase (P less than 0.01) in the packing density between LH + 2 and LH + 6; this is likely to be due, at least in part, to the glands filling up with secretory products and so compressing the intervening stroma. A substantial decrease (P less than 0.01) was seen in the packing density between LH + 6 and LH + 8. This corresponds to the time when stromal oedema is thought to be maximal.