OBJECTIVE To determine whether successive cycles of clomiphene citrate affect endometrial thickness. METHODS Thirty-four women presenting for treatment of anovulation, oligoovulation, or follicle maturation defects were given the smallest dose of clomiphene citrate necessary to attain a mature follicle. If no pregnancy ensued, the same dose was continued if a follicle 18-24 mm in diameter and a serum estradiol (E2) level greater than 200 pg/mL were achieved. Ethinyl E2 was supplemented for poor cervical mucus only. Endometrial thickness and echo patterns were measured each cycle at peak follicular maturation. RESULTS There was no difference in mean endometrial thickness during the first six cycles of therapy, nor was there a trend for thickness to increase or decrease with successive cycles with or without the addition of ethinyl E2. There was no change in the distribution of echo patterns with successive cycles of clomiphene citrate. Post-treatment measures of thickness and echo pattern did not differ from baseline pre-treatment values. The homogeneous hyperechogenic pattern was the rarest. Mean serum E2 and progesterone levels at mid-cycle did not change with successive cycles. CONCLUSIONS One proposed mechanism for the dichotomy between ovulation and pregnancy rates after clomiphene citrate therapy is that the drug adversely affects the endometrium. If clomiphene citrate does affect implantation adversely, the mechanism does not seem to be related to thinning the endometrium or causing an echo pattern that indicates a poor prognosis. The data also suggest that estrogen supplementation does not influence endometrial thickness and would best be used exclusively for hostile cervical mucus.