A bone island, also known as an enostosis, is a common incidental finding (found in passing when looking for something else) of a small area of dense bone inside the spongy part of the bone. Enostoses are common, present in up to 14% of individuals.The cause of bone islands is not known. They are most likely congenital (present from birth) or developmental in nature and are thought to represent either hamartomatous lesions (non-cancerous tumorlike malformations made up of an abnormal mixture of cells and tissues found in areas of the body where growth occurs) or failure of osteoclastic activity (the body's process of breaking down bone in order to build it up again) during bone remodeling. Bone islands are usually considered benign (non-cancerous), stable, nonprogressive lesions, with a preference for the long bones and the pelvis. They typically present without symptoms, do not cause pain and usually do not grow larger. Although bone islands have a characteristic appearance in the vast majority of cases, larger lesions may sometimes pose a diagnostic dilemma, particularly in the setting of known malignancy.