This paper reports on a study of twenty-seven cases of adamantinoma (so-called). Twenty-four of these, including two from the Mayo Clinic, were encountered in a review of the literature, while the other three consisted of new Mayo Clinic cases reported in detail in this paper. In this series, the age of the patients ranged from twelve to fifty-seven years. The tumor involved the tibia in twenty-four cases, the femur in one, the ulna in one, and the fibula in one. Pain was the principal symptom. A mass and swelling were present in a few cases. Fibrous dysplasia coexisted in five cases. Extracranial adamantinoma is a slow-growing epithelial tumor which occurs in bone. The origin of the epithelial tissue is unknown. No agreement has been reached as to whether the epithelial tumor is a basal-cell or a squamous-cell carcinoma. There has been no sex predominance. Microscopically, variations may include the following patterns: basal cells, adenocystic basal cells, squamous cells, an ameloblastic or a sarcomatoid pattern. The treatment of choice is amputation, preferably through the more proximal bone. This opinion is based on the fact that the tumor is almost certain to recur. In seventeen of the twenty-seven cases in this series, recurrence is known to have taken place, followed by death in eight instances. Metastasis was proved by biopsy in four cases, further emphasizing the necessity for radical surgical treatment.