A 54-year-old woman complained of abdominal pain and watery diarrhea that had begun two days before admission. Physical examination revealed abdominal distention and tenderness throughout the abdomen, but there was no muscle guarding or rebound tenderness. The leukocyte count was 27.0 x 10(3)/microl, and CRP was 28.5 mg/dl. A plain film of the abdomen revealed the shadow of a huge gas-containing mass with an air-fluid level and a dilated small intestine with air-fluid level. CT showed a huge gas-containing cystic mass with fatty component and solid structure. Omental thickening was also noted. Emergency surgery was performed, and an enlarged left ovary that was adherent to the small intestine was removed. Microscopic examination revealed a squamous cell carcinoma in the dermoid cyst wall. The carcinoma had directly invaded the small intestine and a fistula between the cyst and the intestine was noted. Thickened omentum showed granulomatous inflammation in the fatty tissue, but no metastases were detected. The histopathological diagnosis was dermoid cyst with malignant transformation and invasion of the small intestine. Chemotherapy was performed, but the patient died of progression of peritoneal metastases 10 months after the operation.