Objective: To evaluate the relationship between uterineDoppler flow and endometrial and subendometrial blood flows during stimulated and natural cycles. Design: A prospective observational study. Setting: A tertiary- assisted reproduction unit. Patient(s): Infertile patients undergoing IVF treatment. Interventions: A three- dimensional ultrasound examination with power Doppler was performed on the day of egg retrieval in stimulated cycles and the day after the LH surge in natural cycles. Main Outcome Measure(s): Pulsatility and resistance indices of uterine vessels, and the vascularization, flow, and vascularization flow indices of endometrial and subendometrial regions. Result(s): Uterine pulsatility and resistance indices were negatively correlated with subendometrial vascularization, flow, and vascularization flow indices in both stimulated and natural cycles, whereas uterine resistance index was negatively correlated with endometrial vascularization and flow indices in natural cycles only. Subendometrial vascularization and vascularization flow indices were significantly lower in patients with an uterine resistance index < 0.95 than those with an uterine resistance index < 0.95. Conclusion(s): Uterine blood flow is a poor reflection of subendometrial blood flow during stimulated and natural cycles, and its measurement cannot reflect endometrial blood flow during stimulated cycles. Objective: To evaluate the relationship between uterine Doppler flow and endometrial and subendometrial blood flows during stimulated and natural cycles. Design: A prospective observational study. Setting: A tertiary- assisted reproduction unit. Patient (s): Infertile patients undergoing IVF treatment. Interventions: A three-dimensional ultrasound examination with power Doppler was performed on the day of egg retrieval in stimulated cycles and the day after the LH surge in natural cycles. Main Outcome Measure (s): Pulsatility and resistance indices of uterine vessels, and the vascularization, flow, and vascularization flow indices of both endometrial and subendometrial regions. Results (s): Uterine pulsatility and resistance indices were negatively correlated with subendometrial vascularization, flow, and vascularization flow indices in both stimulated and natural cycles, but uterine resistance index was negatively correlated with negative endometrial vascularization and flow indices in natural cycl es only. Subendometrial vascularization and vascularization flow indices were significantly lower in patients with an uterine resistance index <0.95 than those with an uterine resistance index <0.95. Conclusion (s): Uterine blood flow is a poor reflection of subendometrial blood flow during stimulated and natural cycles, and its measurement can not reflect endometrial blood flow during stimulated cycles.