We report a case of a clinically and radiographically dominant osteosarcoma of the proximal end of the tibia, in which the second skeletal focus of osteosarcoma was apparent on MRI, after having escaped detection on routine radiographs and scintigraphy. Detection of the second lesion significantly altered the subsequent treatment and prognosis. The improved sensitivity of MRI may increase the accuracy of the diagnosis of osteosarcomatosis. Although further study is needed, MRI study of long bones, especially T1-weighted images in the coronal plane, may be considered as a screening procedure to exclude occult skeletal involvement in patients with osteosarcoma and negative standard metastatic evaluation.