A review of the clinical, radiologic, and histologic features of 25 cases of long bone adamantinoma is presented. To answer some questions concerning the nature of these tumors, relevant tissue markers were analyzed in seven cases using immunohistochemical assays. This study confirmed the epithelial nature of long bone adamantinomas irrespective of their wide-ranging morphologic patterns that can mimic tumors of various origin. On the basis of distinct radiologic, histologic, and immunohistochemical patterns, a new type of adamantinoma termed "differentiated adamantinoma" could be distinguished from the classic long bone adamantinomas. The diagnostic features characteristic of the differentiated adamantinoma include: patient age (first two decades), intracortical location of the entire lesion, uniform predominance of an osteofibrous dysplasia-like pattern, and scattered positivity of epithelial elements for cytokeratin. We postulate that the predominance of an osteofibrous dysplasia-like pattern in differentiated adamantinoma is the result of a secondary reparative process overgrowing matured and regressing tumor tissue. It is possible that this process may lead to the total elimination of recognizable tumor cells from the lesion. Therefore, osteofibrous dysplasia (ossifying fibroma) of long bones, which has a similar anatomic location, age distribution, and radiologic appearance as differentiated adamantinoma, may, in some cases, represent the evolution of an underlying adamantinoma. Our analysis suggests that long bone adamantinoma could be another member of the unique family of tumors that may regress spontaneously.