A case of juxtacortical osteogenic sarcoma of the femur is reported. The patient was a seventeen year-old male who had symptoms of severe pain, swelling on the anteromedial surface of the left lower thigh, limitation of knee motion and a large 15X10X10 cm. sized hard nonmovable tender tumor mass on the lower half of the left thigh for a duration of four months in contrast to the usual mild symptoms seen in the majority of this type of sarcoma. Roentgenographic examination revealed a diffuse soft tissue swelling, a large oval cyst-like lobulated mass without an obvious osteogenic appearance in the interior on the posteromedial aspect of left lower thigh, generalized bony sclerosis of the lower half of the femur, Codman's triangle and partial cortical destruction on the medial aspect of the midpart of the femur. There was no involvement to the distal femoral epiphysis. In blood chemistry there was some elevation of serum alkaline phosphatase (13.7 Bodanski Units). Serum calcium, serum phosphorus and acid phosphatase were within normal values. Diagnosis was made from deep biopsy which revealed histologically a highly malignant osteogenic sarcoma of the femur which may have originated from the juxtacortical area. It was treated by disarticulation of the left hip joint. Histological findings again revealed infiltration of large bizzare malignant spindle cells with plump nuclei and cellular pleomorphism, numerous mitotic figures, giant cells and sometimes multinucleated giant cells. There were, however, no gross osteogenic figures in the tumor.