The standardization of ADHD ratings in adults is important given their differing symptom presentation. The authors investigated the agreement and reliability of rater standardization in a large-scale trial of atomoxetine in adults with ADHD. Training of 91 raters for the investigator-administered ADHD Rating Scale (ADHDRS-IV-Inv) occurred prior to initiation of a large, 31-site atomoxetine trial. Agreement between raters on total scores was established in two ways: (a) by Kappa coefficient (rater agreement for each item with the percentage of raters that had identical item-by-item scores) and (b) intraclass correlation coefficients (reliability). For the ADHDRS-IV-Inv, rater agreement was moderate, and reliability, as measured by Cronbach's alpha, was substantial. The data indicate that clinicians can be trained to reliably evaluate ADHD in adults using the ADHDRS-IV-Inv.