Fragile X syndrome is the most frequent cause of inherited mental retardation; it is caused by expansion of CGG repeats in the first exon of the FMR1 gene. Number of CGG repeats varies between 6 and 50 triplets in normal individuals and the most common alleles have 29 or 30 repeats. Allelic patterns in the global population are similar; however, some reports show statistical differences among several populations. Distribution of allelic frequencies for FMR1 locus has not been reported in Mexican population. Determination of the CGG repeat number was achieved by polymerase chain reaction (PCR) on modified DNA from 129 unrelated Mexican mestizos (46 FRAXA-negative males with mental retardation and 83 healthy individuals). DNA modification by sodium bisulfite achieves conversion of unmethylated cytosine residues to uracil, which allows efficient amplification by single PCR. Methylation status of FMR1 region for each individual was also established. DNA sequencing of a number of amplified samples was realized to validate the procedure. Molecular analysis of the FMR1 gene showed 23 different alleles. Statistical comparison of allelic length between healthy and affected individuals does not show significant differences. Trinucleotide repeat number varied from 16-40, with modal number of 32 (27.58%), second peak at 30 (25.28%), and minor peak at 34 (10.34%). Together, allelic distribution in the Mexican sample differs significantly from those reported for Caucasian, Chinese, African, Indonesian, Brazilian, Chilean, and Mixtec populations. An excess of large alleles (> or =34 repeats) was evident. Allele distribution in FMR1 gene from Mexican mestizos is different from that of other reported populations around the world. This unusual modal pattern probably is related to the particular ethnic background of the Mexican population. On the other hand, PCR on modified DNA is a valuable and efficient method for determination of CGG repetitive sequences in FMR1 gene.