This study tested the incremental validity of behavioral observations, over and above parent and teacher reports, for assessing symptoms of Attention Deficit/Hyperactivity Disorder (ADHD) in children ages 6 to 12, using the Test Observation Form (TOF) and Direct Observation Form (DOF) from the Achenbach System of Empirically Based Assessment. The TOF Attention Problems and DOF Intrusive scales contributed significant unique variance, over and above parent and teacher ratings, to predicting parent and teacher ratings of hyperactivity and impulsivity and predicting categorical diagnoses of ADHD-Combined type versus Non-ADHD and ADHD-Combined type versus ADHD-Predominantly Inattentive type. The TOF Oppositional and Attention Deficit/Hyperactivity Problems scales contributed unique variance to predicting parent ratings of hyperactivity and impulsivity and the DOF Oppositional and Attention Deficit/Hyperactivity Problems scales contributed unique variance to predicting teacher ratings of hyperactivity and impulsivity. (Contains 7 tables and 1 footnote.)