Autism Spectrum Disorders (ASD) are most likely multifactorial diseases in which the combination of genetic and environmental factors might have a role in the expression of the phenotype. A high incidence of gastrointestinal (GI) symptoms is reported in ASD. GI disturbances and altered gut microflora could make a child with a genetic predisposition for ASD more prone to express the ASD phenotype or increase the severity of his behavioral symptoms. The exploitation of strategies which can reduce the gut production and absorption of toxins or restore normal gut microbiota, such as probiotics may represent a non-pharmacological option in the treatment of GI disturbances in ASD. The aim of this study is to determine effects of probiotics supplementation with Vivomixx® in ASD children on specific GI symptoms, ASD core deficits, cognitive and language development, on inflammatory and gastrointestinal (GI) biomarkers and on Quantitative Electroencephalographic measures (QEEG). Vivomixx® is a probiotic mixture of 8 probiotic strains (Bifidobacterium breve, Bifidobacterium longum, Bifidobacterium infantis, Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillus paracasei, Lactobacillus bulgaricus and Streptococcus thermophilus). An additional aim of the study is to determine the environmental exposure to phthalates (chemical pollutant) in ASD children, and the possible effects of probiotic supplementation on their urinary concentrations. A group of 100 unselected preschoolers with ASD will be classified as belonging to the Gastro Intestinal (GI) group or to the Not Gastro Intestinal (NGI) group on the basis of the presence of significant GI symptoms at GI severity Index. Subjects belonging to the two groups (GI and NGI) will be blind randomized 1:1 to regular diet with probiotic Vivomixx® or with placebo for 6 months. All the participants will be assessed at the baseline, after three months and after six months from the baseline in order to evaluate the possible changes in GI symptoms, in ASD symptomatology, in other affective and behavioral comorbid symptoms, in plasmatic, urinary and fecal biomarkers related to abnormal intestinal function and in the electrophysiological patterns. The effects of treatments with probiotics on children with ASD need to be confirmed by rigorous controlled trials. Aiming to examine the impact of this treatment not only on clinical but also on neurophysiological patterns this trial sets out to provide new insights into the gut-brain connection in autism. Moreover, this study's results could add new data on the relationship between the presence of phthalates, clinical features and neurophysiological patterns in ASD.