The possible antiestrogenic effect of clomiphene citrate on endometrial growth was assessed by serial transabdominal ultrasonographic measurements in 46 women with unexplained infertility. A significantly higher (p < 0.05) rate of endometrial growth was observed in the 8 women who conceived compared with the 38 women who failed to conceive. A thicker endometrium (4.6 +/- 0.3 mm) was observed from the third day before ovulation as well as a higher average growth rate in the three days prior to ovulation (0.7 +/- 0.1 mm/day) in the group that achieved pregnancy compared to a lower endometrium (2.1 +/- 0.2 mm) from the third day before ovulation and a slower growth rate in the three days prior to ovulation (0.2 +/- 0.0 mm/day) in the group that did not achieve pregnancy (p < 0.05). These data suggest that clomiphene citrate may exert an antiestrogenic effect on the endometrium, and thus interfere with implantation.