Technetium-99m methylene diphosphonate bone scans were performed in 21 patients with giant cell tumors of bone. All tumors showed increased radiophosphate uptake, often more intense at the tumor periphery than in its center. However, radionuclide bone scanning often overestimated intraosseous tumor extent as a result of increased tracer uptake beyond true osseous tumor limits. In addition, it failed to detect soft-tissue tumor extension in nine patients. Therefore, scintigraphy is less useful than either computed or conventional tomography in planning surgical margins of giant cell tumors. Gallium-67 citrate scans obtained in seven patients showed slight uptake in four tumors and no uptake in three. Radiogallium imaging is thus of limited use in evaluation of suspected giant cell tumors of bone.