Ovarian hyperstimulation syndrome (OHSS) is a serious complication affecting ovulation induction. Its most severe manifestation takes the form of massive ovarian enlargement and multiple cysts, haemoconcentration and third-space accumulation of fluid. The full-blown clinical syndrome may be complicated by renal failure and oliguria, hypovolaemic shock, thromboembolic episodes, adult respiratory distress syndrome (ARDS), and death. Although the pathophysiology of this syndrome has not been completely elucidated, it seems likely that the increased capillary permeability triggered by the release of vasoactive substance secreted by the ovaries under human chorionic gonadotrophin (HCG) stimulation plays a key role in this syndrome. Several factors such as histamine, serotonin, prostaglandins, prolactin, and a variety of other substances have been implicated in this process in the past. At present, factors belonging to the renin-angiotensin system, cytokines including the interleukins, tumour necrosis factor alpha, endothelin-1 and vascular endothelial growth factor (VEGF) are thought to be involved in triggering increased vascular permeability after ovulation induction treatment. This manuscript summarizes the current knowledge of the pathophysiology of ovarian hyperstimulation syndrome with emphasis on the correlation of the various factors with the clinical phenomena of this iatrogenic syndrome.