This study examines clostridia and para-cresol in individuals with ASD. Initial research studies demonstrate a significant increase in urinary p-cresol concentrations among ASD patients compared with matched controls. Urinary p-cresol cannot derive from human metabolism; it can either stem from the presence of clostridium difficile in the gut or from toxic contamination with p-cresol-containing hydrocarbon mixtures. Excess urinary p-cresol may originate from abnormal gut flora or excessive gut permeability in children with ASD. This study intends to define the degree of overlap between elevated urinary p-cresol, clostridial gut infection, and enhanced intestinal permeability. The correlation between urinary p-cresol and a clinical history of regression will also be examined in study subjects. Finally, preliminary experiments will be conducted on approximately 12 young and adult rodents by injecting p-cresol to assess whether and to what extent p-cresol can influence the development and/or function of the central nervous system.