We retrospectively analysed plain radiographs and MR imaging of 16 conventional osteosarcoma cases(8 children, 8 adults) which underwent amputations or limb salvage operations. Pathologic correlation was performed with gross and microscopic sections to evaluate 1) whether the open epiphyseal plate can function as a barrier against transphyseal spread of osteosarcoma and 2) the diagnostic value of MR imaging for the detection of the epiphyseal involvement of osteosarcoma. In children with open epiphyseal plates, conventional radiographs suggested transphyseal tumor growth in one of eight cases(12.5%) and MR imging in seven cases(87.5%). Pathologic examination confirmed epiphyseal involvement in six of seven cases noted with MR imaging (75%). On the other hand, in adult patients with closed epiphyseal plates, conventional radiographs showed transphyseal tumor growth in six of eight cases(75%), while MR imaging and pathologic exam demonstrated tumor invasion in all cases(100%). W conclude that open epiphyseal plate does not function as and effective barrier against tumor extension, and MR imaging is an excellent method in detecting the extent of transphyseal tumor growth.