Inattention and impulsivity symptoms are common among adults with attention-deficit/hyperactivity disorder (ADHD), which can lead to difficulty concentrating, restlessness, difficulty completing tasks, disorganization, impatience, and impulsiveness. Many adults with ADHD find it difficult to focus and prioritize. Resulting outcomes, such as missed deadlines and forgotten engagements, may ultimately impact the ability to function at work, school, home, or in a social environment. The European Medicines Agency guidelines for evaluating medicinal products for ADHD recommend inclusion of both functional outcomes, such as school, social, or work functioning, and outcomes related to symptoms of ADHD in clinical studies of novel medication primary efficacy endpoints. Due to its performance in other disease areas and the relevance of its items as evidenced by content validity analyses, the Sheehan Disability Scale (SDS) was chosen to assess functional impairment in ADHD. The aim of this study was to investigate the psychometric properties of the SDS, used as a brief measure of functional impairment in a number of psychiatric disorders, in adult patients with ADHD. To the authors' knowledge, this is the first study to evaluate the reliability of the SDS (based on Cronbach's coefficient alpha and test-retest reliability), its validity (construct and known-groups validity), and its ability to detect change in this patient population. This study also established a preliminary responder definition for the SDS in this study population to determine when change can be considered clinically beneficial in a clinical trial setting. The psychometric results support the use of the SDS subscales (items 1-3) and total score (sum of items 1-3) in an ADHD population. In addition, the evaluation provides evidence for a three-point preliminary responder definition for the SDS and further evidence of its responsiveness in adults with ADHD. Altogether, the results indicate that the SDS is a simple and easy-to-score scale that would have great utility in future clinical trials for monitoring functional impairment in adults with ADHD.