Forty-five cases of osteosarcoma were studied for transepiphyseal spread of the lesion because of the current interest in local resection and chemotherapy as treatment of this disease. In 17 cases, the epiphyseal plate had closed and all 17 had transepiphyseal extension of the tumor. In 28 cases, the epiphyseal plate was open; 21 showed growth or microscopic evidence of transepiphyseal extension of the lesion. The most common method of extension was directly through the epiphyseal plate along vascular channels, or less commonly, about the epiphyseal plate beneath the perichondrium and into the epiphysis along the epiphyeal vascular channels. The majority of these extensions were not detectable by either conventional radiography or radioisotope scanning. Local resection with preservation of the epiphysis will, in all likelihood, leave residual disease despite the oft-quoted statement that an open epiphyseal plate is a biologic barrier to the extension of bone tumors.