The hallmark of McCune-Albright syndrome is premature puberty in the female. Menstrual periods may begin in early childhood, long before the appearance of breast or pubic hair development, which normally occur first. Early sexual development may also occur in males, but this is not as common as in females.

These children have abnormal fibrous dysplasia (abnormal fibrous development) in the bone that can lead to fractures, deformities, and abnormal x-rays. Abnormal bone growth in the skull can lead to blindness or deafness due to "pinched" nerves. Deformities may appear in the facial bones and cause cosmetic problems. In addition, the children have cafe-au-lait spots on their skin, which may be present from birth.

McCune-Albright syndrome is caused by mutations in the GNAS1 gene. It is associated with mosaicism, meaning that the abnormal gene is present in a fraction, but not all, of the patient's cells. This disease is not inherited, it is sporadic. This means that a new mutation of the DNA occurs in the womb during development of the patient. This mutation is not passed on to any children the patient later has.