To study follicular and endometrial growth patterns in Clomiphene Citrate (CC) stimulated cycles, 50 CC cycles of 31 infertile women with patient Fallopian Tubes, were followed, 17 spontaneous conceptional cycles of fertile women were followed as controls. The pattern of follicular growth was similar in both groups until the day before ovulation in which CC cycles showed larger follicular diameters than spontaneous ones; 23.8 +/- 3.1 mm versus 21.6 +/- 2.9 mm (p = 0.013). Follicular rupture occurred on day 16.1 +/- 2.9 in CC cycles, and on day 15.1 +/- 1.85 on spontaneous conceptional ones. This suggests that the follicle, under the influence of CC, has to reach a larger critical mass to produce enough estradiol to revert the hypothalamic blockage produced by the drug, thus permitting the preovulatory LH surge. Endometrium under CC action, was always thinner than in natural cycles. On the day of follicular rupture, the CC cycle's endometrium measured 11.1 +/- 2.02 mm and the natural cycle's endometrium measured 10.6 +/- 1.8 mm (ns). This finding could be attributed to a antiestrogenic effect of CC on the endometrium. It is concluded that cycles under CC action have different follicular and endometrial growth patterns than spontaneous conceptional cycles.