Objective : Clomiphene citrate is one of the most commonly used drugs in the treatment of infertility, but the pregnancy rate achieved with clomiphene citrate is significantly lower than the ovulation rate due to its antiestrogenic effect on the endometrium. Endometrial prolactin is considered to be a marker and an inducer of predecidualization that is characteristic of secretory endometrium. The purpose of this study was to evaluate the association of clomiphene citrate and unsatisfactory endometrial differentiation to secretory endometrium by examining the endometrial expression of prolactin in clomiphene citratetreated infertile women with luteal phase defect. Methods : The endometrial samples from infertile women with luteal phase defect (n=27) were examined. Five cases during secretory phase and six cases during proliferative phase were obtained by biopsy. Sixteen cases were obtained by biopsy during secretory phase after clomiphene citrate treatment. By immunohistochemical staining for prolactin, all obtained endometrial tissues were examined. The differences in the endometrial expression of prolactin were evaluated between proliferative phase and secretory phase, and between clomiphene citrate treated group and no treatment group during secretory phase. Results: The staining of endometrial prolactin was significantly more intense in the glandular epithelial cells and stromal cells in the secretory endometrium than in the proliferative endometrium. The glandular expression of prolactin in the secretory endometrium was not significantly different between the clomiphene citrate-treated group and no treatment group (p=0.719), but the staining of prolactin in the stromal cells was significantly less intense in the clomiphene citrate-treated group than no treatment group (p=0.019). Conclusion: In this investigation, we demonstrated that the endometrial stromal expression of prolactin in the secretory phase was significantly lower in the clomiphene citrate-treated group campared with no treatment group in infertile women with luteal phase defect. And our finding suggests that clomiphene citrate may have an adverse effect on the endometrial predecidualization in infertile women.