Osteofibrous dysplasia (OFD) and adamantinoma are rare and most commonly arise in the tibia of young individuals. Although OFD has typical histopathologic features, areas resembling OFD have often been noted at the periphery of otherwise classic adamantinomas, and some have suggested that OFD may be either a precursor to or a regressive phase of adamantinoma. The so-called OFD-like adamantinoma encompasses some features of both OFD and adamantinoma. We studied the clinical, imaging, histopathologic, immunohistochemical, ultrastructural, and molecular features of 16 OFD and 8 adamantinomas (5 OFD-like and 3 classic) in an attempt to further define their morphology, clinical course, and relationship. Patients with OFD were generally younger than those with adamantinoma. Osteoblastic and osteoclastic activity was more prominent in OFD than in OFD-like adamantinoma. In addition to the inconspicuous small clusters of epithelial cells in OFD-like adamantinoma, isolated keratin-positive cells with a unique ultrastructural hybrid fibroblastic-epithelial phenotype were found in the stroma of all OFD and OFD-like adamantinomas. Fluorescence in situ hybridization analysis revealed trisomies 7, 8, and/or 12 in the spindle cell stroma of OFD, OFD-like, and classic adamantinoma, supporting a neoplastic origin of OFD and a common histogenesis for all 3 lesions. Trisomies were not observed in osteoblasts or osteoclasts suggesting that the osseous component is reactive and non-neoplastic. Of the 11 OFD patients with follow-up (median, 4.5 y), all 3 who underwent incisional biopsy had persistent, nonprogressive disease and 2 of 8 who underwent curettage or wide excision had recurrence; none developed adamantinoma. All 6 adamantinoma patients with follow-up (3 classic and 3 OFD-like) were treated with wide excision. One with classic adamantinoma died of pulmonary metastases 9 years after presentation; the other 5 were free of disease with a median follow-up of 12 years. None of the classic adamantinomas evolved into OFD-like adamantinoma or OFD. Although the histopathology, immunohistochemistry, ultrastructure, and cytogenetics indicate that these lesions are closely related, our data and the literature suggest that only classic adamantinoma has malignant potential. OFD, OFD-like adamantinoma, and classic adamantinoma appear to show a progressive complexity of cytogenetic aberrations, perhaps indicative of a multistep neoplastic transformation.