The accurate pre-operative evaluation of the intramedullary extent of osteosarcoma is essential, as it determines the level of bone resection. Radiographs, isotope bone and MR imaging scans have been considered as reliable in detecting skeletal metastasis and skip lesions. We report a case of osteosarcoma of the distal femur with a large skip lesion proximally which was not visualized by either routine radiography or bone scintigraphy, and was not included within the scan field on the initial MR imaging scan. The implications on patient management and possible reasons for failure of imaging to reveal the skip metastasis are discussed.