A 38-year-old woman was transferred to the hospital with lower abdominal pain. Magnetic resonance (MR) imaging revealed an irregular ovarian wall with a solid component and a fluid-fluid level in the cystic mass and the pelvic space, which was thought to be pathognomonic for the rupture of an endometrioma with a malignant ovarian tumor. Histologic examination following adnexectomy revealed a ruptured endometrioma associated with endometrioid adenocarcinoma. A fluid-fluid level in the cystic mass and pelvic space may be pathognomonic MRI feature for a rupture of either an endometrioma or an endometrioma with a malignant tumor.