Background: We evaluated the imaging features of ovarian teratomas containing fluid–fluid levels on ultrasonography (US). Methods: We retrospectively reviewed US examinations of two groups with 805 masses (370 benign ovarian teratomas and 435 nonteratomatous adnexal masses). Results: In 27 teratomas and eight nonteratomatous adnexal masses, fluid–fluid levels were detected on US. According to the echogenicity of each layer, 27 teratomas were classified as three types: 1, supernatant hypoechoic and dependent hyperechoic layers (n= 16); 2, supernatant hyperechoic and dependent hypoechoic layers (n= 8); and 3, supernatant hypoechoic and dependent hypoechoic layers with bright fluid interface (n= 3). In eight (30%) of 27 teratomas, US showed floating nodules at the interface, five of which had posterior acoustic shadowing. All eight nonteratomatous adnexal masses showed type 1 fluid–fluid levels. Conclusion: The fluid–fluid level seen on US is strongly suggestive but not pathognomonic of dermoids. Fluid–fluid levels with supernatant hyperechoic and dependent hypoechoic layers, supernatant hypoechoic and dependent hypoechoic layers with bright interface, and a floating nodule might pathognomonic findings of benign ovarian teratomas.