Polyostotic fibrous dysplasia: A disorder that features the replacement of multiple areas of bone by fibrous tissue, which may cause fractures and deformity of the legs, arms, and skull. A genetic disorder that is characterized by polyostotic fibrous dysplasia along with skin pigmentation and hormonal problems, with premature sexual development, is known as McCune-Albright's syndrome. The flat areas of increased skin pigment are called caf' au lait spots. The hormonal problems that can be related to polyostotic fibrous dysplasia include early puberty (with premature menstrual bleeding and development of breasts and pubic hair), thyroid abnormalities, and an increased rate of growth. Also known as McCune-Albright syndrome.