We examined three microsatellites in the arginine vasopressin 1a receptor gene (AVPR1a), two in the promoter region (RS1 and RS3) and an intronic microsatellite (AVR), for association with autism as well as scores on the Vineland Adaptive Behavior Scale (VABS), the Autism Diagnostic Interview-Revised (ADI-R) and the Autism Diagnostic Observation Scale-Generic (ADOS-G), measures that are widely used to diagnose autism spectrum disorders. We tested for association between the AVPR1a microsatellites and autism in 116 families (128 probands diagnosed with the ADI-R and ADOS-G using a family-based association test (UNPHASED)). Testing each individual microsatellite showed significant transmission disequilibrium in these families with the AVR intronic microsatellite (UNPHASED: LRS=11.46, global P-value=0.009, df=3). Haplotype analysis of three microsatellites also showed significant association (LRS=144.94, df=103, global P=0.004). Additionally, significant association is observed between these three microsatellite haplotypes and the VABS scores (P=0.009), with the ADI-R (P=0.009) and the ADOS-G (P=0.0000765) diagnoses of autistic disorder versus pervasive developmental disorder-not otherwise specified (PDD-NOS) that were available for 47 of these probands. This is the third consecutive report of an association between the AVPR1a gene and autism spectrum disorders and in the current study a third microsatellite is shown to be associated with autism spectrum disorders as well as haplotypes consisting of all three markers. Importantly, the association appears to be mainly mediated by the role of the AVPR1a gene in shaping socialization skills, similar to its role in lower vertebrates.Molecular Psychiatry (2006) 11, 488-494. doi:10.1038/sj.mp.4001812; published online 7 March 2006.