Doppler studies of the uterine artery using an off-set Doppler transducer with a mechanical sector imaging transducer indicate clear changes in uterine perfusion during the ovarian cycle. In this study, 16 volunteers had Doppler studies performed at least twice weekly during spontaneous ovarian cycles. Endocrine assays were performed on each occasion to measure plasma oestradiol, progesterone and luteinizing hormone levels. Conventional criteria to assess uterine impedance using systolic/end diastolic ratio and Resistance Index were modified to obtain meaningful results and a new flow velocity wave form classification is presented. The results indicate increasing uterine perfusion with rising levels of plasma oestradiol and progesterone and a direct correlation with falling oestrogen levels in the follicular phase. We conclude that off-set mechanical sector duplex systems can be used effectively to monitor uterine responses to the hormone environment.