Endometriosis associated with ascites is an uncommon condition, but nevertheless important because it mimics the presentation of ovarian malignancy. We present the case of a 34-year-old woman who was admitted with the acute onset of abdominal pain and distension. A pelvic sonogram showed a large amount of peritoneal fluid with echogenic material and a 10-cm complex cystic structure, which appeared to be an enlarged left ovary. The concentrations of CA125 and CA19-9 were 548.1 and 7604 IU/mL, respectively. On the second day of admission, an exploratory laparoscopy was performed, which revealed a ruptured left ovarian cyst consistent with an endometrioma. A laparoscopic left adnexectomy was therefore performed. The histological examination of the specimen confirmed the diagnosis of an endometrioma. After a 12-month follow-up period, the patient showed no recurrence of the endometrioma or peritoneal fluid accumulation. In most instances, the presence of massive ascites is associated with malignancies, tuberculosis, or a perforated viscus. The information provided in the present report is important to both gynecologists and oncologists because it shows that an ovarian cyst with ascites and highly elevated CA125 and CA19-9 levels might be benign.