Objective: To present a case of severe ovarian hyperstimulation syndrome (OHSS) in a naturally conceived singleton pregnancy after ovulation induction. Case: A 31-year-old woman with polycystic ovarian syndrome (PCOS) underwent ovulation induction therapy. Six days later, she was admitted to the present hospital with the symptoms of OHSS. Ultrasonography confirmed the single live intrauterine pregnancy, as well as enlarged multicystic ovaries and marked ascites that required abdominal paracentesis. The woman was treated with intravenous infusion, exogenous colloid supplementation, essentiale liver treatment, rocephin anti-infective therapy for skin lymphangitis of left lower limb, and drainage of the ascites. She recovered by day 45 of admission. Conclusion: Severe OHSS may develop in women with PCOS who undergo ovulation induction therapy. Serious complications may develop rapidly and therefore OHSS must be treated urgently and with multidisciplinary management.