Objective: This study addresses if deficits in cognitive, attention, and inhibitory control performance in adults with ADHD are better explained by the disorder itself or by comorbid conditions. Method Adult patients with ADHD ("n" = 352) and controls ("n" = 94) were evaluated in the ADHD program of a tertiary hospital. The diagnostic process for ADHD and comorbidities was based on "Diagnostic and Statistical Manual of Mental Disorders" (4th ed.; "DSM-IV") criteria. Stepwise regression analyses evaluated the effect of ADHD, demographics, and comorbidities on the scores from Wechsler Adult Intelligence Scale-Revised, Continuous Performance Test, and Stroop Color and Word Test. Results: Patients with ADHD of both genders had worse performance on neuropsychological domains, even after adjustment for comorbidities. The presence of comorbid bipolar disorder and specific phobia are associated with more Stroop errors, whereas patients with generalized anxiety disorder present a longer execution time in Stroop. Conclusion: Neuropsychological deficits in adults with ADHD go beyond comorbidity. Specific comorbid disorders may influence the neuropsychological functioning in adults with ADHD. (Contains 2 tables.)