The investigators propose to test the hypothesis that defects in mitochondrial function may be an important factor in ASD pathophysiology. Physiological and biochemical evidence of mitochondrial dysfunction will be directly assessed by collecting skeletal muscle, assaying for mitochondrial function and quantifying oxidative stress. Subjects will be further evaluated for mitochondrial dysfunction using five non-invasive techniques, which have the potential to supplant the muscle biopsy in diagnosis of mitochondrial dysfunction. These non-invasive tests include analysis of alterations in metabolite levels in urine and blood (metabolomics), determination of changes in brain and muscle high energy phosphates by magnetic resonance spectroscopy (MRS), determination or patient exercise capacity and resulting alterations in hormone levels, analysis of exhaled organic molecules, and analysis of changes in mitochondrial status by transdermal near infrared light absorbance. To determine if alterations in mitochondrial genes contribute to the development of ASD, the investigators will screen subjects for alterations in mitochondrial genes that are located in chromosomal regions previously implicated in autism. Analyses will also include mtDNA copy number and changes in the mtDNA sequence that may correlate with disease. What this means for people with autism:Demonstration that individuals with ASD have mitochondrial defects could provide important insights into both regressive and progressive aspects of the clinical course of ASD in children and adults. In sum, this research program is in position to develop novel methods for diagnosis of metabolic abnormalities and offer immediate treatment approaches.