Adamantinomas of the long bones are low-grade malignant tumours. They seem to be related to osteofibrous dysplasia with a mesenchymal-to-epithelial transformation. We report a case of an adamantinoma with a revertant sarcomatoid transformation that showed a complete loss of epithelial differentiation. It corresponded to a 41-year-old male presented with an 8-cm multilobated lesion in the centre of the distal tibia. On the en bloc resection specimen, areas of classic adamantinoma were found but most of the tumor corresponded to a high-grade neoplasm with 2 histologic patterns: one made up by epithelial nests with a basaloid arrangement and positive for pankeratins and so-called glandular keratins, and a second sarcomatoid component, negative for these epithelial markers. Five months after surgery the patient had a massive relapse that consisted solely of the second sarcomatous component also negative for epithelial antibodies.Three cases of adamantinomas with sarcomatoid transformation of the epithelial component have been described but the tumours still preserved an epithelial immunophenotype. However, our case represents the extreme end of the sarcomatoid dedifferentiation of a classic adamantinoma with complete sarcomatoid transformation and total loss of epithelial markers. To our knowledge this has not been described previously.