Fragile X syndrome , which is caused by expanded CGG repeats of the FMR1 gene , is associated with a broad spectrum of clinical involvement and is the most common inherited form of intellectual disability. Early diagnosis and intervention are likely to lead to improved outcome for children with fragile X syndrome , but such strategies require better estimates of the frequencies of expanded alleles of the FMR1 gene. In this study , we report the results of a newborn screening study of 5267 male blood spots collected from the Northwest region of Spain as part of the national newborn screening program. The blood spots were screened using a rapid polymerase chain reaction-based method that is capable of identifying the presence of all expanded alleles for both males and females. The screened samples included 199 gray zone alleles , 21 premutation alleles , and two full mutation alleles (1 in 2633). The frequency of premutation alleles was three times higher (1 in 251) than the quoted value of 1 in 813 from a Canadian population and is fully consistent with the results of large-scale Israeli screening studies. Our results demonstrate that newborn screening for the presence of expanded FMR1 alleles is an effective means for defining the distribution of expanded FMR1 alleles in newborn populations; as such , this method is suitable for large-scale newborn screening.