Objective: Validation of the Adult ADHD Investigator Symptom Rating Scale (AISRS) that measures aspects of ADHD in adults. Method: Psychometric properties of the AISRS total and AISRS subscales are analyzed and compared to the Conners' Adult Attention-Deficit/Hyperactivity Disorder Rating Scale-Investigator Rated: Screening Version (CAARS-Inv:SV) and the Clinical Global Impression-ADHD-Severity Scale using data from a placebo-controlled 6-month clinical trial of once-daily atomoxetine. Results: The AISRS has high internal consistency, good convergent, and discriminant validities; modest divergent validity; and small ceiling and floor effects ([greater than or equal to] 1%). It correlates highly with the CAARS-Inv:SV. Factor analysis confirms 2 AISRS subscales, hyperactivity-impulsive scale and inattention. The AISRS total and AISRS subscales perform stably. All scales demonstrate responsiveness to change with medication. Conclusion: The AISRS and its subscales are robust, valid efficacy measures of ADHD symptoms in adult patients. Its anchored items and semistructured interview are advancements over existing scales. (Contains 6 tables and 2 figures.)