Giant Endometrioma of the Ovarian Mimicking Malignant Tumor

18F-FDG PET/CT findings of a giant ovarian endometrioma are exceedingly rare. We present a case of a 27-year-old woman with a giant pelvic cystic mass mimicking malignancy. 18F-FDG PET/CT revealed a massive cystic lesion occupying the entire abdominopelvic cavity, with mild internal septations and mild FDG uptake. Despite elevated tumor marker levels, weight loss, and suspicious imaging findings, the final histopathologic diagnosis was bilateral ovarian endometriomas.