Two adult patients with multiple hereditary exostoses, a skeletal disorder with recognized malignant potential, each demonstrated increased 99mTc diphosphonate uptake in an exostosis in which renewed growth had begun. None of the other multiple exostoses in either patient showed abnormal uptake. Histologic study of the lesions demonstrated chondrosarcoma in one case and benign osteochondroma in the second. Although bone scintigraphy nonspecifically identifies bone growth rather than malignant degeneration, it is more useful than radiographic bone survey in the periodic surveillance of adult patients with this disorder.