Objective: The present research examined the validity of self-report versus informant-report in relation to a performance-based indicator of adult ADHD. Method: Archival data from 118 participants (52 males, 66 females) were used to compare Conners’ Adult ADHD Rating Scale–Self-Report: Long Format (CAARS-S:L) and Conners’ Adult ADHD Rating Scale–Observer Report: Long Format (CAARS-O:L) with discrepancy scores calculated between the Wechsler Adult Intelligence Scale–Third Edition (WAIS-III) Verbal Comprehension Index – Working Memory Index (VCI – WMI) and Perceptual-Organizational Index – Processing Speed Index (POI – PSI) scaled scores. Results: Neither the self- nor informant-report formats of the CAARS were better predictors of discrepancies between WAIS-III Index scores. Intercorrelations between the CAARS-S:L and CAARS-O:L revealed generally higher correlations between the same scales of different formats and among scales measuring externally visible symptoms. Furthermore, regression analysis indicated that both the CAARS-S:L and CAARS-O:L clinical scales contributed a significant proportion of variance in WAIS-III VCI – WMI discrepancy scores (14.7% and 16.4%, respectively). Conclusion: Results did not establish greater accuracy of self-report versus informant-report of ADHD symptomatology, rather demonstrate the need for multimodal assessment of ADHD in adults. (J. of Att. Dis. 2013; XX(X) 1-XX)