Unsuspected chronic rupture was discovered during surgical removal of a very large ovarian dermoid cyst. Abdomen X-ray, US and CT examinations were preoperatively performed. Standing X-ray projection of the abdomen allowed the appearing of a new radiographic finding of dermoid cyst, the "fat floating" as the equivalent of the "gravity dependent layering" in US and CT features. This radiographic sign appears as an horizontal line between two soft tissues of different opacity; it is caused by oily and sebaceous fluid floating over serous fluid and over intracystic debris. Literature was reviewed and radiographic findings in dermoid cysts were reconsidered; the sign of "fat floating" could enhance the diagnostic accuracy of abdomen X-ray. So when a pelvic or an abdominal-pelvic mass is discovered in a young woman, standing projection is required for abdomen X-ray. Rupture of a dermoid cyst may happen without notice and chronically; CT has been more accurate than US in evaluating rupture, in particular the peritoneal spread of oily pseudocyst.