OBJECTIVE To study the changes that occur in vascular resistance to flow in the utero-ovarian circulation and their correlation with the number of follicles and steroid hormone levels in patients undergoing ovarian stimulation with gonadotropin. METHODS In a prospective study, the impedance to flow in the intraovarian and uterine vessels was measured by means of transvaginal color flow Doppler imaging throughout the cycle. METHODS University-based IVF program. METHODS Eleven infertile patients undergoing ovarian stimulation with gonadotropin in preparation for IVF and ET. METHODS Intraovarian, uterine, and arcuate artery blood flow velocity waveforms, follicular growth, and serum E2 and P concentrations throughout the cycle. RESULTS Pulsatility index (PI) of the intraovarian blood vessels and uterine artery decreased gradually during the follicular and luteal phase. The PI of the arcuate artery did not change significantly. The PI of the intraovarian blood vessels correlated with the number of follicles (> 15 mm; day of hCG). Serum E2 concentrations but not P demonstrated negative linear correlation with the PI of the intraovarian vessels and uterine artery. CONCLUSIONS Induced cycles are associated with decreased impedance to blood flow in the utero-ovarian circulation. Intraovarian PI correlates negatively at each stage of the cycle with the eventual number of preovulatory follicles. Little or no changes are observed when the response to ovarian stimulation is poor.