Clinical features of Chilean patients with Fibrous Dysplasia/McCune-Albright SyndromeBackground: Fibrous Dysplasia/McCune-Albright Syndrome (FD/MAS) is characterized by a spectrum of manifestations that may include fibrous dysplasia of bone and multiple endocrinopathies. Aim: To describe the clinical spectrum, the study and follow-up of patients with FD/MAS cared at our institution. Material and Methods: Review of medical records of 12 pediatric and adult patients (11 women) who met the clinical and genetic diagnostic criteria for FD/ MAS. Results:The patients' mean age at diagnosis was 4.9 ± 5.5 years. The most common initial clinical manifestation was peripheral precocious puberty (PPP) in 67% of patients and 75% had café-au-lait spots. Fibrous dysplasia was present in 75% of patients and the mean age at diagnosis was 7.9 ± 4.7 years. Ten patients had a bone scintigraphy, with an age at the first examination that varied between 2 and 38 years of age. The most frequent location of dysplasia was craniofacial and appendicular. No patient had a recorded history of cholestasis, hepatitis, or pancreatitis. In four patients, a genetic study was performed that was positive for the pathogenic variant of guanine nucleotide binding protein, alpha stimulating (GNAS). Conclusions: These patients demonstrate the variable nature of the clinical presentation and study of FD/MAS. It is essential to increase the index of diagnostic suspicion and adherence to international recommendations.