The aims of this study were 1) to compare the prevalence of overweight and obesity between children and adolescents with ASD treated with risperidone with the general pediatric population and 2) to investigate the association of genetic polymorphisms of the target receptor gene HTR2C –759C>T (rs3813929) and efflux transporter gene ABCB1 1236C>T (rs1128503), 2677G>T/A (rs2032582) and 3435C>T (rs1045642) with overweight/obesity in children and adolescents with ASD treated with risperidone. This evidence concerns the gene ABCB1 and obesity due to melanocortin 4 receptor deficiency.