Thirty cases of cortical osteofibrous dysplasia (COFD) were studied in an attempt at defining the relationship of COFD to adamantinoma. The patients ranged in age from newborn to 39 years (mean 13.4 years). The male:female ratio was 1:1. Presenting symptoms were most often pain or a mass. The tibia was involved in all 30 patients; in addition, the ipsilateral fibula was involved in five patients (17%). The histologic appearance of the lesions was dominated by the combination of woven bone trabeculae with prominent osteoblastic rimming and a loose, slightly myxoid stroma (less heavily collagenized in most instances than usually encountered in intramedullary fibrous dysplasia). Results of immunohistochemical study showed isolated cytokeratin-positive cells in the stroma of 28 of the lesions (93%). However, hyperchromatic epithelial islands characteristic of adamantinoma were not found in any of the 30 cases. A control population of 50 fibro-osseous lesions (intramedullary fibrous dysplasia, sclerosing fibroxanthoma, and cranial ossifying fibroma) was studied immunohistochemically; in none of these control cases were cytokeratin-positive cells found. Follow-up data were obtained in 17 cases (57%); the period ranged from 1 to 16 years (mean 6.05 years). Certain overlapping clinical features (including the location of the vast majority of the lesions in the tibia and, less often, the fibula) and the morphologic similarities of many areas of COFD and adamantinoma (particularly the shared presence of cytokeratin-positive cells) suggest a more than coincidental association between COFD and a adamantinoma. However, to date none of the 30 cases of COFD evaluated in this study has developed an adamantinoma.