The authors observed 2 cases of fibrous dysplasia on the cranial and maxillofacial bones in 31, 28 aged sisters, who had come to the Infirmary of Dental College, Yonsei University. The serial roentgenograms and clinical findings had been taken and the results established as polyostotic fibrous dysplasia according to the findings in their images. The authors have obtained the results as follows: 1. Bony expansion of the mandible occured at 18 years of age and the facial assymetry appeared due to development of the lesions. 2. The traumatic history were not noted but weak tendency of familial history noted 3. Endocrine disturbances, hyperpigmentation on the skin and premature puberty in the infancy were not noted. 4. We have concluded these diseases as polyostotic fibrous dysplasia on the cranial and maxillofacial bones with weak familial tendency according to the findings