Objective: To identify similarities and differences in neuropsychiatric correlates in children with Tourette's syndrome (TS) and those with ADHD. Method: The sample consisted of children with Tourette's syndrome with ADHD(N=79), children with Tourette's syndrome without ADHD (N=18), children with ADHD (N=563), psychiatrically referred children (N=212), and healthy controls (N=140). Results: Disorders specifically associated with Tourette's syndrome were obsessive compulsive disorder (OCD) and simple phobias. Rates of other disorders, including other disruptive behavioral, mood, and anxiety disorders, neuropsychologic correlates, and social and school functioning were indistinguishable in children with Tourette's and ADHD. However, children with Tourette's syndrome plus ADHD had more additional comorbid disorders overall and lower psychosocial function than children with ADHD. Conclusions: These findings confirm previously noted associations between Tourette's syndrome and OCDbut suggest that disruptive behavioral, mood, and anxiety disorders as well as cognitive dysfunctions may be accounted for by comorbidity with ADHD. However, Tourette's syndrome plus ADHD appears to be a more severe condition than ADHD alone.