The neuropsychological functioning of adults with Attention Deficit Hyperactivity Disorder (ADHD) was compared to that of healthy controls and individuals with mild psychiatric disorders including attentional complaints. Thirty adults in each group were examined on the Conners' Continuous Performance Test (CPT) and measures of attention, executive function, psychomotor speed, and arithmetic skills. The ADHD group performed lower than healthy controls on most measures. However when compared to the psychiatric group, the performances of the ADHD group were not significantly lower on any of the measures. The predictive power of the tests was poor in discriminating ADHD from psychiatric disorder. Implications for the clinical diagnosis of ADHD are discussed.