In the present study, researchers will test the hypothesis that early changes in brain growth in autism has long-term consequences for neural connectivity, or communication between different regions of the brain, as changes in connectivity are thought to underlie the symptoms of autism. Dr. Kleinhans and colleagues will examine this proposed relationship between early brain growth and later brain connectivity. They will gather previously collected measurements of brain size and volume from 3 to 5 year old children with low-functioning autism. When these children are 15 years old, they will be asked to participate in a brain imaging study using functional magnetic resonance imaging (fMRI), a non-invasive method of imaging patterns of brain activity and neural connectivity. These data should determine whether an increase in brain size during childhood is correlated with weakened or changed patterns of connections between brain regions in teenagers with autism and contribute to the understanding of the developmental basis of behavioral impairment and clinical outcomes in autism spectrum disorders.