There have been several causes of infertility attributed to gamete quality, congenital anatomical abnormalities and surgical complications. Published research into the reasons for failure of implantation of embryos has been confined to histochemical and histological studies of the endometrluin. This paper presents preliminary data from an ongoing study to test the hypothesis that poor uterine perfusion is a cause of failure of Implantation of embryos. It would follow that poor uterine perfusion is a cause of infertility. One-hundred- and-fifty-three patients who had been unsuccessful in conceiving despite three previous In-vitro fertilization attempts have been studied. Doppler ultrasound studies of the ascending branch of the uterine artery, during spontaneous ovarian cycles, revealed a poor mid-secretory uterine response In 48% of patients studied. Patients with poor mid-secretory uterine response were treated with orally administered hormone therapy to improve the mid-secretory uterine response prior to subsequent embryo replacement. The results of subsequent in-vitro fertilization therapy in patients with good uterine response and in women with hnproved uterine response after hormone therapy are presented. The numbers of patients in each group are insufficient for statistical analyses, but the trends observed support the hypothesis that poor uterine blood flow is a cause of infertility. Further evaluation is warranted.