A case of osteosarcoma is described that originated within the cortex of the tibial diaphysis. This form of osteosarcoma was first described by Jaffe as "intracortical osteogenic sarcoma." A review of the literature indicates that this is probably the rarest presenting form of osteosarcoma. It appears to have a predilection for origin in the diaphysis, and its radiologic appearance may mimic a benign lesion. Follow-up information is given on the two cases originally reported by Jaffe, and the tumor is distinguished from periosteal osteosarcoma with which it has recently been confused.