A series of 17 patients with osteosarcomas that histologically resembled osteoblastomas was studied. The ages of the 9 male and 8 female patients ranged from 11 to 58 years. The roentgenographic appearance was suggestive of malignancy in most cases. Two histologic features seemed most important in differentiating osteosarcoma from osteoblastoma. In the former, there is permeation of surrounding tissues and lack of "maturation" toward the edges, whereas osteoblastoma tends to show maturation peripherally and is circumscribed. Osteoblastoma-like osteosarcoma should be considered to be a malignant tumor because 7 of the 17 patients died of their disease. The authors believe that malignant osteoblastoma and aggressive osteoblastoma are really osteosarcomas that resemble osteoblastomas.