We describe a case of ovarian hyperstimulation syndrome (OHSS) complicated by peritonitis due to perforated appendicitis. A 29-year-old woman presented with abdominal distension after ovarian stimulation with HMG followed by ovulation induction with HCG. Massive ascites with swollen ovaries was observed on ultrasound, and she was admitted on the diagnosis of OHSS. Daily infusion of serum albumin and low dose dopamine failed to increase her urine output and her abdominal symptoms became increasingly deteriorated after her urine pregnancy test turned out to be positive. Paracentesis performed for alleviation of her abdominal distension revealed infected, foul-smelling fluid. An emergency laparotomy was performed, and the definite diagnosis was made as panperitonitis due to perforated appendicitis with right tubal pregnancy. Appendectomy, right tubectomy and vigorous irrigation with drainage were performed. The case implies that OHSS might not only mask typical manifestations of appendicitis, but could also compromise concurrent intraperitoneal infection.