BACKGROUND: Ovarian hyperstimulation syndrome (OHSS) is one of the most serious complications of ovulation induction by exogenous gonadotropins. The pathophysiologic factors of this syndrome are not well known. Increased capillary permeability causes third space fluid shift, which is responsible for ascites, pleural fluid, and edemas. Severe OHSS may result in renal failure, hypovolemic shock, thromboembolic disease, respiratory distress, and may cause death. It has been observed that paracentesis is efficacious, provided that care is taken to reinfuse protein lost in the peritoneal exudate. For this reason, in three patients with severe OHSS we have used a dialytic technique of reinfusion of concentrated ascitic fluid. STUDY DESIGN: We treated three patients with severe OHSS (grade 6). Through sonography-guided paracentesis, the ascitic fluid was concentrated by ultrafiltration and reinfused. This treatment was instituted and performed once only. Ultrafiltration was obtained with a common high-flow dialyzer (polyacrylonitryle membrane). The concentrated fluid was returned to the patient in a peripheral vein. We have limited further treatment to restoration of fluid and electrolyte balance, avoiding in particular potentially teratogenic drugs. RESULTS: In all three patients, a progressive increase of diuresis was evident during treatment and subjective improvement was almost immediate. Fifteen days after treatment, hematologic and biochemical parameters had returned within normal limits. CONCLUSIONS: In treating severe OHSS, we have used the technique of reinfusion of concentrated ascitic fluid to avoid protein depletion induced by paracentesis. We have been able to successfully restore to normal the hematologic and biochemical imbalance with one treatment. Use of the technique described herein should be limited to carefully selected instances and treatment should be performed in an intensive care unit.