The hallmark symptom of McCune-Albright syndrome is premature puberty in girls. Menstrual periods may begin in early childhood, long before the breasts or pubic hair develop (which normally occur first). Puberty and menstrual bleeding may begin as early as 4 - 6 months in girls.

Early sexual development may also occur in boys, but it is not as common as it is in girls.

Other symptoms include:

• Bone fractures
• Deformities of the bones in the face
• Gigantism
• Irregular, large patchy café-au-lait spots, especially on the back

A physical examination may show signs of:

• Abnormal bone growth in the skull
• Abnormal heart rhythms (arrhythmias)
• Acromegaly
• Adrenal abnormalities
• Gigantism
• Hyperparathyroidism
• Hyperthyroidism
• Hypophosphatemia
• Large café-au-lait spots on the skin
• Liver disease, jaundice, fatty liver
• Ovarian cysts
• Pituitary or thyroid tumors
• Scar-like tissue in the bone (fibrous dysplasia)

Tests may show too much:

• Adrenal hormones
• Blood prolactin
• Growth hormone

Other tests that may be done include:

• MRI of the head
• X-rays of the bones

Genetic testing is available for the GNAS1 gene.