Reports of increasing prevalence of autism spectrum disorders (ASDs) are intensifying interest in the role of environmental exposures, including infectious, immune, and toxic factors. This prospective study will employ the Autism Birth Cohort (ABC), a smaller cohort (ASD, N=150-233) selected from the Norway Mothers and Child (MoBa) study, which consists of 75,500 Norwegian mother-child pairs. In the ABC study, children meeting criteria for an autism diagnosis are ascertained through screening of the entire population with diagnoses established using uniform procedures. In addition to an autism diagnosis, extensive histories and clinical data are obtained and biologic samples are collected serially throughout pregnancy and early childhood. Researchers in this study aim to identify gene-environment factors contributing to the development of autism, including: 1) immune, endocrine, and neuroregulatory factors in mothers during early gestation or at birth, 2) environmental factors, including pre- or postnatal infection, vaccination, very low birth weight or other obstetric risk factors, dietary and/or environmental exposure to methylmercury, and 3) genotypic influences that are associated with a family history of autoimmune disease or other ASD candidate genes. The Norwegian ABC study offers an unprecedented, rich view of the longitudinal trajectory and nascent signs and symptoms of autism spectrum disorders, facilitating discovery of biomarkers and gene-environment interactions in ASD pathogenesis.