The ovarian hyperstimulation syndrome is the most important iatrogenic complication of ovarian stimulation. Every drug used in the treatment of infertility such as FSH, HMG, CC, GnRHa, can lead to the syndrome. The authors of the article report the incidence of OHSS in their patients treated for an assisted conception program; the role of oestrogens, HCG and renin-angiotensin system; the classification, the therapy and how to prevent the syndrome.