We compared symptoms of generalized anxiety disorder (GAD) and separation anxiety disorder (SAD) in 5 groups of boys with neurobehavioral syndromes: attention-deficit/hyperactivity disorder (ADHD) plus autism spectrum disorder (ASD), ADHD plus chronic multiple tic disorder (CMTD), ASD only, ADHD only, and community Controls. Anxiety symptoms were assessed using parent and teacher versions of a DSM-IV-referenced rating scale. All three groups of boys with co-morbid ADHD evidenced more severe anxiety than Controls. Group differences in anxiety varied as a function of symptom, disorder, informant, and co-morbidity supporting the notion that co-morbid neurobehavioral syndromes differentially impact clinical features of co-occurring anxiety symptoms. Findings also suggest that GAD and SAD are phenomenologically unique, even in children with ASD. Implications for nosology are discussed.