In a tri-institutional, retrospective study with long-term follow-up, forty-four patients who had multiple enchondromas were identified. Thirty-seven patients did not have hemangiomas (Ollier disease) and seven did (Maffucci syndrome). Of the thirty-seven patients who had Ollier disease, a low-grade chondrosarcoma developed in four; an astrocytoma, in one; and a granulosa-cell ovarian tumor, in one. In four of the seven patients who had Maffucci syndrome, there were six low-grade chondrosarcomas, one high-grade osteosarcoma, one pancreatic adenocarcinoma, one biliary adenocarcinoma, and one astrocytoma. None of the patients in either group died of the skeletal sarcoma, but four of five patients who had a non-skeletal malignant lesion died. From life-table analyses of these patients, we estimated that the incidence of secondary chondrosarcoma in patients who have Ollier disease is about 25 per cent at the age of forty years, and that malignant degeneration is almost a certainty in patients who have Maffucci syndrome. We concluded that periodic surveillance of the brain and abdomen for occult malignant lesions is indicated in patients who have enchondromatosis.