There is little research on the functional assessment and treatment of anxiety and related problem behavior in children with autism spectrum disorder (ASD), particularly those with intellectual and developmental disability (IDD). In a recent study, we evaluated a multimethod strategy for assessing anxiety in children with ASD and IDD (Am J Intellect Dev Disabil 118:419-434, 2013). In the present study, we developed treatments for the anxiety and associated problem behavior in these same children. A multiple baseline design was used to evaluate the effectiveness of a multicomponent intervention package, incorporating individualized strategies from Positive Behavior Support and Cognitive Behavioral Therapy. During intervention, all three participants showed substantial decreases in anxiety and problem behavior and significant increases in respiratory sinus arrhythmia in the situations that had previously been identified as anxiety-provoking.