Background: The present study investigated the impact of coexisting anxiety disorder in children with ADHD on their ability to regulate behavior. Method: Parent reports on the Behavior Rating Inventory of Executive Function (BRIEF) in a comorbid group of children with ADHD and anxiety (n = 11) were compared to BRIEF reports in a group of children with a "pure" ADHD (n = 23), a "pure" anxiety (n = 24) and a group without any diagnosis (n = 104) in a 2 (ADHD vs. no ADHD) x 2 (anxiety vs. no anxiety) design. Results: The children with ADHD and anxiety disorder scored significantly higher on the Inhibit scale than children within the other three groups. Main effects of diagnosis appeared in ADHD children on the Inhibit, Emotional Control, and Working Memory scales, and on the Shift and Emotional Control scales in anxious children. Conclusion: The results indicate that a behavioral dysregulation in ADHD children is aggravated by comorbid anxiety. (Contains 1 table and 4 figures.)