This research investigates relationships between brain structural development and autism symptom onset that typically presents between 12 and 24 months of age. This proposal will add brain chemical information (through the use of 3-D Proton Echo Planar Spectroscopic Imaging, or 3D PEPSI) to assess mechanisms of abnormalities in brain growth (regions, tissues, structures, and fiber tracts) that are measured by magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI). These measures will be related to clinical features of autism. The addition of 3D PEPSI will allow measurements of brain chemistry in 6 month-old siblings of children diagnosed with autism (high risk) and age-matched infants without a family history of autism (low risk). These infants will be rescanned at 12 and 24 months and clinically assessed for a possible autism diagnosis at 24 months. Brain chemical measures collected as part of this project have the potential to help identify distinct clinical subtypes of autism spectrum disorder (ASD) that may have prognostic and treatment implications. Brain chemical measurements will provide biomarkers that may identify distinct ASD endophenotypes and help elucidate underlying pathophysiological mechanisms responsible for abnormalities in brain development in autism.