To investigate ultrastructural modifications in the endometrium of women receiving clomiphene citrate (CC) treatment, we performed endometrial biopsies in the preovulatory phase in 17 patients with anovulatory infertility and 16 normo-ovulating women with tubal or male sterility. Histology, scanning and transmission electron microscopy (SEM and TEM) revealed midproliferative phase aspects in 11.7% of the treated group and 6% of the controls, late proliferative aspects in 76.4% and 82%, and postovulation days 1-3 characteristics in 11.7% and 12% respectively. No significant differences were observed between the patients and controls; our data indicate that CC taken in a dose not exceeding 100 mg/day from day 3 of the cycle can induce endometrial maturation that is almost identical with that of a spontaneous preovulatory phase. This type of endometrium should therefore allow implantation of a fertilized egg and maintain unmodified the complex biohumoral relations with the embryo.