Parosteal osteoid sarcoma, sometimes referred to as“juxtacortical osteogenic sarcoma”, is a rare malignant bone tumor which must be differentiated from an osteosarcoma clinically, roentgenologically, and histologically. Its relatively benign course was described by Muller as early as 1843 (13), but it was not before 1947 that parosteal osteoid sarcoma, representing only 1 per cent of all bone tumors, was recognized as a distinct and separate entity (9). This tumor occurs most frequently in the third and fourth decades of life and is without sex predilection. It involves the large cylindrical bones of the extremities, predominantly the metaphysis (Fig. 1). The distal femur is affected most often. The tumor usually presents a palpable hard mass which does not adhere to the skin. It is frequently associated with pain and limitation of motion. Symptoms may have been present for months or even years. All parosteal osteoid sarcomas are basically malignant tumors (Fig. 2) which if untreated eventually metastasiz...