Twelve patients with histologically proved sarcoma arising in fibrous dysplasia of bone (six monostotic, six polyostotic) are described. In five (Group I) the existence of the dysplasia was known years before the sarcoma arose, and in seven (Group II) the discovery of both lesions was concurrent. The average age of patients in Group I
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was fifteen years when compared with thirty-five years in Group II. In only one instance could the history of previous radiotherapy be elicited. The histological examination of the primary bone sarcomas revealed eight osteosarcomas, two chondrosarcomas, and two spindle-cell sarcomas.