Objective: Wechsler Intelligence Scale for Children Freedom-from-Distractibility/Working Memory Index (FDI/WMI), Processing Speed Index (PSI), and Gordon Diagnostic System (GDS) scores in ADHD children were examined as a function of subtype and coexisting anxiety, depression, and oppositional-defiant disorder. Method: Participants were 587 children with ADHD combined type (alone, with oppositional-defiant disorder, and with anxiety or depression) and ADHD inattentive type (alone and with anxiety or depression). Results: Attention scores (FDI/WMI and GDS Vigilance and Distractibility) were similar and low for all groups. ADHD combined groups had greater impulsivity (GDS Delay) than ADHD inattentive groups, and ADHD inattentive groups had slower processing speed (PSI) than ADHD combined groups. Comorbidity did not affect test performance. Scores differentiated ADHD subtypes with 72% accuracy. Conclusion: Findings support the validity of "DSM-IV" ADHD combined and inattentive subtypes and the validity of the GDS, FDI/WMI, and PSI in assessing attention, impulsivity, and processing speed in children with ADHD. (Contains 2 tables.)