Autism is a severe neurodevelopmental disorder defined by social and communication deficits and ritualistic-repetitive behaviors that are detectable in early childhood. Brain-derived neurotrophic factor (BDNF) plays a critical role in the pathogenesis of autism. In this study, we examined the SNP- and haplotypic-association of BDNF with autism in a trios-based association study (the Autism Genetic Resource Exchange). We also examined the expression of BDNF mRNA in the peripheral blood lymphocytes of drug-naive autism patients and control subjects. In the TDT of autism trios, the SNP haplotype combinations showed significant associations in the autism group. BDNF expression in the drug-naive autistic group was found to be significantly higher than in the control group. We suggest that BDNF has a possible role in the pathogenesis of autism through its neurotrophic effects on the serotonergic system. (c) 2007 Elsevier Inc. All rights reserved.