The antiestrogenic action of clomiphene citrate (CC) is claimed to have an adverse effect on the development of the secretory endometrium. This effect can be assessed: (1) sonographically by measuring endometrial thickness, and (2) by serum hormone levels. The aim of this study was to evaluate whether administering ethinyl estradiol (EE) during CC treatment has any effect on endometrial thickness and/or hormone levels. Seventeen patients were treated with CC for one cycle and with CC plus EE in an adjacent cycle either before or after. The patients were followed by daily assessment of endometrial thickness, follicular growth and serum estradiol levels as well as midluteal prolactin and progesterone levels. We did not find any significant difference in either endometrial thickness, estradiol level, midluteal prolactin or progesterone levels between the two treatment protocols. We conclude that exogenous EE, in the dosage used in this study, does not overcome CC-induced alterations in endometrial thickness.