The objective of this study was to define the incidence of febrile morbidity and its causes in severe and critical ovarian hyperstimulation syndrome (OHSS), For this purpose, we reviewed the medical records of ail OHSS patients hospitalized in 16 out of 19 tertiary medical centres in Israel between January 1987 and December 1996, Febrile:morbidity was defined as at least one episode of temperature rise above 38 degrees C lasting greater than or equal to 24 h. A total of 2902 patients (3305 hospitalizations) with OHSS was identified, of whom 196 had severe, and 13 critical, OHSS, Among the 209 patients investigated the incidence of febrile morbidity was 82.3%, of which 20.5% was attributed to urinary tract infection, 3.8% to pneumonia, 3.3% to upper respiratory tract infection, 2.0% to intravenous line phlebitis, 1.0% to cellulitis at an abdominal puncture site, 1.0% to postoperative wound infections and 0.5 % to gluteal abscess at the site of progesterone injection. Non-typical organisms were frequently isolated, such as Pseudomonas, Proteus, Klebsiella and Enterobacter species. No infectious aetiology was found in 105 patients (50.2%). Hypoglobulinaemia was recorded in most patients, while ascitic and pleural fluids aspirated from these patients contained high globulin concentrations, We conclude that infection-related febrile morbidity in severe and critical OHSS is high, and may be attributed to some degree of immunodeficiency associated with loss of plasma globulins to the third space. However, non-infection-related febrile morbidity is even higher and may be attributed to endogenous pyrogenic mechanisms.