Gastrointestinal (GI) dysfunction is relatively prevalent in children with ASD, but is especially difficult to diagnose, not only because of the social and communication deficits which prevent effective reporting of symptoms, but also the difficulties clinicians face in correlating behavior with specific GI disorders. Current clinical practice guidelines include neither routine GI evaluation nor diagnostic criteria or protocols that consider the special gastrointestinal needs of this patient population. Therefore, the first objective of the proposed study is to develop and validate effective and appropriate diagnostic screening methods to detect symptomatic GI dysfunction in children with ASD. The second objective of the proposed research is to determine whether gastrointestinal (GI) dysfunction contributes substantially to the expression of problem behaviors (PB) in children with ASD and to determine, if specific behaviors, including but not limited to PB, are increased in those children with gastrointestinal dysfunctions. The main hypothesis to be tested is that painful or discomfort-causing gastrointestinal (GI) dysfunction occurs frequently in children with ASD, and contributes to an elevated incidence or severity of PB in an identifiable subpopulation of these children. What this means for people with autism: The results of the study will inform the medical community about the prevalence of GI symtpoms as they relate to autism and will provide evidence that will inform clinical practice guidelines for children with ASD, thereby improving quality of life for these young patients and their family members.