Objective: To report an unexpected case of severe ovarian hyperstimulation syndrome (OHSS) and to compare E(2) levels and number of follicles to other oocyte donors. Setting: Private assisted reproduction technology center. Patients: Healthy oocyte donors with normal menstrual cyclicity. Interventions: Prophylactic and therapeutic use of human serum albumin infusions. Main Outcome Measure: The clinical development of signs and symptoms of severe OHSS. Results: More than 60% of other oocyte donors had higher E(2) levels and 12% had higher number of follicles without associated OHSS. Conclusion: The risk of developing severe OHSS cannot be predicted accurately to be low even in the absence of ''risk factors.''