Adult participants for the current study were placed into one of two groups depending on whether they manifested either high or low symptom levels of Attention Deficit/Hyperactivity Disorder (ADHD) as determined by the Wender Utah Rating Scale. Participants were also assessed on a battery of cognitive tasks as well as symptoms of depression, anxiety and substance abuse. The ADHD symptom groups were comparable in age and symptoms of depression, anxiety, and substance abuse. Adults with greater ADHD symptomatology performed more poorly on cognitive measures of response inhibition (the Stop-Signal task) and visual attention, compared to those with fewer ADHD symptoms. Regression analysis indicated the Stop-Signal task accounted for a greater proportion of the variance of ADHD symptomatology than any other cognitive variable. The conclusions from the findings are discussed in relation to Barkley's (1997) self-regulation model of ADHD.