Eighteen nonmedicated adults with attention-deficit hyperactivity disorder (ADHD) and 18 who did not have ADHD were evaluated in a full-day simulated workplace experience. It was hypothesized that adults with ADHD would evidence greater impairments on simulated tasks, off-task behavior, and self-reported ADHD symptoms than those without ADHD. Participants were compared on self-reported ADHD symptoms, objective observations, and performance on written tasks. Significant differences were noted in reading comprehension and math fluency as well as observer-rated and self-reported behavior, but not attention. The results of this study suggest that ADHD among adults is associated with significant deficits in performance of workplace tasks, internal experiences, and external observations of core symptoms of ADHD.