Objective: To evaluate how distinct presentations of anxiety symptoms and intellectual impairment influence the measurement and estimated rate of clinically significant anxiety in autism spectrum disorder (ASD). Method: The sample included 75 children (ages 9-13 years) with ASD and varied IQ and 52 typically developing (TD) controls and parents. Parents completed anxiety symptom scales and a diagnostic interview, designed to (1) differentiate anxiety and ASD and (2) examine DSMspecified and unspecified ("distinct") anxiety presentations in each child, including fears of change, special interests, idiosyncratic stimuli and social confusion rather than evaluation. Children completed standard intellectual and ASD diagnostic assessments. Results: 69% of those with ASD had clinically-significant anxiety, including 21% DSM-specified anxiety disorders, 17% distinct anxiety, and 31% both. Only 8% of TD children had clinicallysignificant anxiety, all DSM-specified. DSM-specified anxiety disorders in children with ASD and intellectual impairment (IQ<70) were predominantly specific phobias. DSM-specified anxiety other than specific phobia was significantly less common in children with, versus without, intellectual impairment; this was not the case for distinct anxiety. The sensitivities of anxiety scales were moderate to poor, particularly in cases with intellectual impairment. Conclusions: ASD is associated with more frequent and varied presentations of clinical anxiety, which may align with and differ from the specified anxiety disorders of the DSM. Standard parent report anxiety scales have reduced sensitivity to detect clinical anxiety in ASD, particularly in children with intellectual impairment. "This summer [1937] we brought him to a playground slide and on the first afternoon when the other children were sliding on it he would not get about it, and when we put him up to slide down it he seemed horrorstruck. The next morning when nobody was present, however, he walked out, climbed the ladder, and slid down and he has slid on it frequently since, but slides only when no other child is present to join him in sliding." Case 1, "Donald" "He frets when the bread is put in the oven to be made into toast, and is afraid it will get burned and be hurt. He is upset when the sun sets. He is upset because the moon does not always appear in the sky at night." Case 8, "Alfred"