The present study investigated the construct validity of the Internal Restlessness Scale (IRS), a self-report instrument developed to measure feelings of restlessness in young adults with ADHD, and the relationship between the IRS and a neuropsychological, behavioral task (i.e., continuous performance test). Pearson product-moment correlations indicated that the IRS correlated significantly with the self-report rating scales (both those specifically measuring ADHD symptoms and those measuring overall psychological symptoms and intelligence), but not with behavioral measures purported to measure the constructs of attention and impulsivity. Furthermore, the correlations between the IRS and other ADHD rating scales were significantly higher than the correlations between the IRS and non-ADHD rating scales. Overall, results support the construct validity of the IRS. (Contains 4 tables.)