Objective: To determine if comorbid anxiety disorder is associated with differential response to immediate release methylphenidate (MPH-IR) in children with both ADHD and chronic multiple tic disorder (CMTD). Method: Children with (n = 17) and without (n = 37) diagnosed anxiety disorder (ANX) were evaluated in an 8-week, placebo-controlled trial with rating scales and laboratory measures. Results: The +ANX group obtained more severe parent, teacher, and child ratings of anxiety and more severe parent ratings of depression, tics, oppositional defiant disorder (ODD), and peer aggression than the -ANX group. Treatment with short-term MPH-IR was associated with improvement in ADHD, ODD, and peer aggression in the +ANX group. When controlling for ODD severity, there were no apparent group differences in therapeutic response to MPH-IR in children plus or minus ANX. There was little evidence that MPH-IR contributed to improvement in anxiety or depression symptoms in the +ANX group. There was some indication that children with comorbid anxiety may differentially experience greater increase in systolic blood pressure (0.5 mg/kg of MPH-IR greater than placebo). Conclusion: Findings suggest that the co-occurrence of diagnosed CMTD+ADHD+ANX represents a particularly troublesome clinical phenotype, at least in the home setting. Comorbid anxiety disorder was not associated with a less favorable response to MPH-IR in children with ADHD+CMTD, but replication with larger samples is warranted before firm conclusions can be drawn about potential group differences. (Contains 2 tables and 1 figure.)