Objective: To evaluate the efficacy of IV administration of human albumin solution for the prevention of severe ovarian hyperstimulation syndrome (OHSS). Design: A prospective, randomized, placebo-controlled study comparing the effects of IV administration of human albumin solution versus sodium chloride 0.9% solution at the time of oocyte retrieval with patients undergoing IVF-ET who are at high risk for the development of severe OHSS. Setting: Specialized assisted reproduction unit. Patients: Thirty-one patients undergoing IVF-ET who had serum E(2) levels of 1,906 pg/mL (>7,000 pmol/L) and multiple follicular development on the day of hCG administration. Interventions: After hCG administration, patients were randomized to receive IV, either 50 g of human albumin diluted in 500 mL of sodium chloride 0.9% or 500 mL of sodium chloride 0.9% at the time of oocyte retrieval. Main Outcome Measures: Ovarian size as measured by pelvic ultrasonography, development of ascites, serum E(2) concentrations during the luteal phase, and results of the IVF-ET cycles. Results: Although no patient who had received human albumin solution developed severe OHSS, there were four such cases in the control group. All four were hospitalized with marked ascites and ovarian enlargement. There were no significant differences between the two groups comparing serum E(2) levels on the day of hCG administration and during the luteal phase, the number of oocytes retrieved, fertilization, and pregnancy rates. Conclusions: Our preliminary results suggest that the administration of human albumin solution may help to prevent the development of severe OHSS in high-risk patients. Further research is needed to assess the potential of this novel approach.