ABSTRACT Given the high prevalence and substantial impact of mental illness on individuals with intellectual disabilities (ID) studies that examine mental health in persons with ID are critical to direct treatment aimed at reduction and, more importantly, prevention of mental illness to improve functioning and quality of life. Fragile X syndrome (FXS) is the leading identified cause of ID with up to 80% of adolescents and adults with FXS meeting criteria for an anxiety disorder. FXS is an ideal model to investigate the combined and interactive effects among biological, behavioral and environmental factors on the emerging trajectory of anxiety in persons with ID. This application “Emergence, Stability and Predictors of Anxiety in Fragile X Syndrome” proposes the first longitudinal prospective developmental study of the early features of anxiety in very young boys with FXS contrasted to boys diagnosed with ASD (non-FXS) and typical controls (TD). We are interested in the age at which initial features of anxiety can be detected and the stability and prognostic value of these early symptoms to diagnostic categorization across two populations associated with ID and at high risk for anxiety. Likewise, we are interested in documenting factors that predict the emergence, stability and severity of anxiety in these two populations.