The present disclosure relates to compositions and methods for treating autism spectrum disorder (ASD) by restoring an ASD patient's gut microbiota. These methods can be used with ASD patient with or without ongoing gastrointestinal symptoms. Provided here is a method for ASD treatment in a subject in need thereof comprising or consisting essentially of administering a therapeutic composition comprising a fecal microbe or a fecal microbiota preparation to the subject. Also provided here is a method comprises administering an antibiotic to a human subject; subjecting the human subject to a bowel cleanse; and administering purified fecal microbiota to the human subject. Further provided are evaluation and quantitative characterization of patient symptom improvements upon treatment described here.