We outline the narrative of a 28-year-old woman who initially presented to the emergency department with vomiting, diarrhoea, abdominal pain and fever. Blood tests revealed significantly raised inflammatory markers and acute renal failure. Initially, this was attributed to gastroenteritis due to a recent foreign travel, but further investigations and radiological imaging revealed a large right ovarian dermoid cyst with a significant amount of free intra-abdominal fluid and small bowel dilation. She underwent laparotomy, which revealed a spontaneously perforated right ovarian dermoid cyst resulting in generalised purulent peritonitis and small bowel obstruction due to bowel adherence at the perforation site. Meticulous adhesiolysis, right salpingo-oophorectomy and extensive peritoneal lavage were performed, with a good postoperative recovery. Spontaneous perforation of an ovarian dermoid cyst, without an associated torsion, is extremely rare, but it should be considered in cases of peritonitis and bowel obstruction of unclear cause with a concomitant finding of a dermoid cyst., Competing Interests: Competing interests: None declared., (© BMJ Publishing Group Limited 2019. No commercial re-use. See rights and permissions. Published by BMJ.)