Anxiety is common in youth with autism spectrum disorder. Despite this common co-occurrence, studies targeting anxiety in this population are hindered by the under-developed state of measures in youth with autism spectrum disorder. Content validity (the extent to which an instrument measures the domain of interest) and an instrument’s relevance to the patient population are key components of measurement development. This article describes the application of qualitative research methods in the initial development of a parent-rated instrument of anxiety symptoms in youth with autism spectrum disorder. Overall, 48 parents of 45 children (aged 3–17 years) with autism spectrum disorder and at least mild anxiety participated in one of six focus groups at two sites (three groups per site). Systematic coding of the focus group transcripts identified broad themes reflecting the situations and events that trigger anxiety in children with autism spectrum disorder, the behavioral manifestations of anxiety in children with autism spectrum disorder, the parent and the child’s own response to anxiety, and broad behavioral patterns that could be associated with anxiety. From the focus group data, investigators generated 52 candidate items for a parent-rating of anxiety in youth with autism spectrum disorder. This report provides a detailed description of these early steps in developing a patient-oriented outcome measure.