Twenty-five infertile women conceived while taking tamoxifen (TMX). Daily serum profiles of 5 of the 25 TMX-induced conception cycles were elucidated and compared with those found in 5 normal cycles. In spite of lower levels of follicle-stimulating hormone and luteinizing hormone during the follicular phases, estradiol concentrations were higher in the TMX-induced conception cycles. It is suggested that this may be due to a direct ovarian effect of TMX as one of its major mechanisms in the course of folliculogenesis. On the other hand, progesterone concentrations on days 6 and 7 during the luteal phases were also higher in the TMX-induced conception cycles. It is suggested that this may be due to a luteotropic influence at the blastocyst stage.