Of 55 patients with Ollier's disease seen at the Mayo Clinic between 1907 and 1985, 16 had malignant bone neoplasms: 12 chondrosarcomas, two dedifferentiated chondrosarcomas, one chordoma, and one osteosarcoma. One patient had a chondrosarcoma in two different bones. These findings suggest that approximately 30% of patients with Ollier's disease will develop a malignant bone neoplasm, most probably chondrosarcoma. The prognosis for most patients is good. Five of the 16 patients survived more than 13 years after treatment.