The purpose of this study was to empirically evaluate the diagnostic utility of the Conners CPT and the P ASAT as a laboratory-based measures of attention, vigilance, working memory, and sustained mental effort in clinic-referred adults with and without ADHD. Moreover, the study was conducted in order to obtain additional evidence in ongoing efforts to clarify the classification and phenomenological status of ADHD in adults. Subjects were adults seeking psychological evaluation for a variety of presenting problems including, emotional, occupational, relational, and/or adjustment, difficulties. Standardized clinical evaluations were conducted and structured clinical interviews were used to determine clinical diagnosis and group membership (ADHD vs. Non-ADHD). It was hypothesized that ADHD adults would exhibit significantly more comorbid psychopathology, learning problems, and verbal memory deficits than non-ADHD adults. It was also hypothesized that the findings from the CPT (Conners, 1992) and P ASAT (Gronwall, 1977) would reliably discriminate these groups (ADHD and non-ADHD) of clinic-referred adults. While ADHD adults did not exhibit a higher frequency of learning disabilities, they did have significantly more comorbid psychopathology and were much more likely to experience clinically impairing deficits in verbal memory functioning. The combined results of the laboratory tasks accurately identified diagnostic group status in over 9 of 10 cases for ADHD adults and in approximately 8 of 10 cases for non-ADHD adults. The relevance of these findings for the psychiatric classification and experiential nature of ADHD in adults, as well as the clinical diagnostic utility of these laboratory measures for adult ADHD is discussed. / Master of Science