A case of fibrous dysplasia of the mandible has been presented in which further diagnostic procedures showed the presence of a second and potentially more dangerous lesion. The literature was reviewed with the purpose of bringing attention to the still confused state of fibro-osseous lesions of the jaws and skeleton. The relationship of the ossifying fibroma and fibrous dysplasia was explored in relative depth. Fibrous dysplasia is a usually benign fibro-osseous abnormality of bone that may occur as monostotic, polyostotic, or craniofacial disease or as a part of a syndrome. Its nosology is confusing and its etiology is still unknown.