Background: McCune-Albright Syndrome (MAS) is characterized by precocious puberty, "cafe au lait" skin lesions and polyostotic fibrous dysplasia. It is caused by 4 post-zygotic mutations of Gas protein with a mosaic distribution. Aim: To describe the clinical presentation and to investigate the presence of the Arg by his substitution (R201H) in 14 girls with MAS. Patients and methods: We performed a clinical analysis of the patients and specific allele PCR in DNA obtained from leukocytes. Results: Twelve of 14 patients presented with precocious puberty, one with cyclical vaginal bleeding and one with pathological bone fractures. Eight girls had polyostotic fibrous dysplasia, one had hyperthyroidism, four had pathological fractures, ten had ovarian cysts, six had breast hyperpigmentation and ten had "cafe au lait" skin lesions. We detected the R2O1H mutation in 10 of 14 patients. We found no difference in the severity of symptoms or in the age of presentation between the patients with and without the mutation. Conclusions: The R201H mutation can be detected in white blood cells, in approximately 70% of cases. Patients exhibit wide clinical variability with the same molecular defect. This suggests that tissues have different proportions of mutant cells (Rev Méd Chile 2001; 129: 1365-72)