Little is known about which clinical features may aid the differentiation between attention deficit hyperactivity disorder (ADHD) and other clinical conditions. This study seeks to determine the role of self- and informant reports on symptoms and impairments in the clinical evaluation of adult ADHD and explore their association with objective neuropsychological test performance by examining data of 169 outpatients referred for a diagnostic evaluation of adult ADHD. Participants were assigned either to an ADHD group (ADHD, n = 73) or one of two clinical comparison groups, depending on whether they show indications (Clinical Comparison Group, CCG, n = 53) or no indications (Clinical Comparison Group—Not Diagnosed, CCG-ND, n = 43) of psychiatric disorders other than ADHD. All participants and their informants completed a set of questionnaires. Compared to the CCG-ND, the ADHD group obtained significantly higher scores on ADHD symptoms, impulsivity, cognitive deficits, and anxiety. Compared to the CCG, the ADHD group scored significantly higher on ADHD symptoms but lower on depression. Further regression analyses revealed that self- and informant reports failed to predict neuropsychological test performance. Self- and informant reported information may be distinct features and do not correspond to results of objective neuropsychological testing.