The objective of this study was to follow the kinetics of four inflammatory cytokines in the plasma and ascitic fluid of seven patients who developed severe ovarian hyperstimulation syndrome (OHSS) after induction of ovulation for in-vitro fertilization. Blood samples were obtained from these patients at three different times: upon hospitalization; when significant clinical improvement was evident; and after complete resolution. Samples were analysed for interleukin-1 (IL-1), interleukin-6 (IL-6), interleukin-8 (IL-8) and tumour necrosis factor alpha (TNF-alpha). Ascitic fluid was obtained by therapeutic paracentesis from all study patients during the active phase and analysed for these cytokines. Two control groups were available: the first included 15 women undergoing controlled ovarian stimulation for in-vitro fertilization without developing OHSS, while the second consisted of 25 healthy women not undergoing ovulation induction or any other medical treatment. High concentrations of IL-1, IL-6 and TNF-alpha were detected in all individuals upon admission for severe OHSS. Concentrations dropped significantly along with clinical improvement, reaching normal values after complete resolution. A statistically significant correlation was found between plasma cytokine concentrations and certain biological characteristics of the syndrome such as leukocytosis, increased haematocrit, and elevated plasma 17-beta-oestradiol concentrations. Ascitic fluid obtained from the study patients contained high IL-6 and IL-8 concentrations, while other cytokines were unaltered. These results suggest close association between inflammatory cytokines and the pathophysiology of the ovarian hyperstimulation syndrome.