Test examiners used the Test Observation Form (McConaughy & Achenbach, 2004) to rate test session behavior of 177 6- to 11-year-old children during administration of the Wechsler Intelligence Scale for Children-Fourth Edition (WISC-IV) and Wechsler Individual Achievement Tests-Second Edition (WIAT-II). Participants were assigned to four groups based on a parent diagnostic interview and parent and teacher rating scales: attention deficit hyperactivity disorder (ADHD)-Combined type (n = 74); ADHD-Inattentive type (n = 25); clinically referred without ADHD (n = 52); and controls (n = 26). The ADHD-Combined type group scored significantly higher than the other three groups on six Test Observation Form scales: (1) Attention Problems; (2) Oppositional; (3) Attention Deficit/Hyperactivity Problems scale; (4) Inattention subscale; (5) Hyperactivity-Impulsivity subscale; and (6) Externalizing. The two ADHD groups also scored significantly lower than controls on all WISC-IV and WIAT-II composites and lower than those clinically referred without ADHD on WISC-IV Working Memory Index and Full Scale Intelligence Quotient. Implications are discussed regarding the discriminative validity of standardized test session observations for identifying children with ADHD and differentiating between the two ADHD subtypes. (Contains 5 tables and 2 footnotes.)