Abstract Objective: To assess intraovarian blood flow in relation to the severity of ovarian hyperstimulation syndrome (OHSS) after controlled ovarian hyperstimulation. Design: A prospective study. Setting: Monash IVF, Clayton, Victoria, Australia. Patient(s): Thirty patients with OHSS after embryo or gamete transfer who also had sonographic evidence of ascites. Main Outcome Measure(s): The resistance to blood flow within the ovaries of 11 patients with severe OHSS and 19 patients with mild OHSS was measured by using transabdominal ultrasonography with color flow and pulsed Doppler imaging. Result(s): The pulsatility index (PI), resistance index (RI), and the S-D ratio, all measures of downstream vascular impedance, were significantly lower in those patients with severe OHSS. In cases with RI < 0.48, more than two thirds of the patients had a pleural effusion, whereas patients with either PI < 0.75 or S-D < 1.92 had pleural effusion in over one half of the cases. It was notable that blood flow velocity did not differ significantly between the two groups despite the changes in vascular impedance. Conclusion(s):There appears to be a close correlation between the severity of OHSS and the resistance to blood flow within the stimulated ovaries. Measurement of intraovarian vascular resistance before gamete transfer or ET in patients undergoing controlled ovarian hyperstimulation may help in predicting those patients at particular risk of developing severe OHSS.