Fragile-X syndrome is due to an expression of CGG trinucleotide repeats in the 5' untranslated region of the FMR1 gene and it is the most common cause of heritable X-linked mental retardation. Until now, the disease and the carrier state were diagnosed by Southern blotting or PCR-based methods. Southern blotting is an expensive, time-consuming, and radioisotope-based method that cannot easily be used for routine screening of an at-risk population. Nonradioisotopic PCR methods do not identify full mutated alleles, nor do they discriminate between alleles in the normal range that differ only by one or two CGG repeats. Therefore, two normal alleles with only a small difference in size, cannot be differentiated after PCR in Metaphor agarose or acrylamide gels. To define the genotype, it is necessary to perform Southern blot analysis. In this paper, we present a new strategy which, because of its simplicity, can be applied to large-scale fragile-X carrier screening of at-risk females.