Autism and Rett syndrome are both pervasive developmental disorders and share many characteristics in common. One of these features is developmental regression with loss of social, cognitive and language skills after a period of apparently normal development during the first 1-2 years of life, which raises the question of whether there is a common pathway underlying regression in these two disorders. The Rett syndrome gene was identified as MeCP2 gene on Xq28, a powerful transcriptional repressor. To explore its possible role in the etiology of autism and involvement in regression, we searched for MeCP2 gene mutations in a well characterized sample of 31 autistic boys with developmental regression by direct sequencing. One sequence variant in 3' untranslated region was observed. The patient inherited the variant from his unaffected mother, so it may be a rare polymorphism. No coding sequence variant was found in any of the patients tested. We conclude that mutations in the coding sequence of MeCP2 are not a frequent cause of regression in autism. The long 3' untranslated region of MeCP2 is highly conserved across species, suggesting that they are important for the post-transcriptional regulation of MeCP2 gene. It may be worthwhile extending the mutation screening, with a larger sample of strictly defined phenotype, to regulatory elements and untranslated regions of this gene, to explore to what degree MeCP2 gene is involved in the etiology of autism and its possible role in the regression of autism.