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Objectives Numerous studies showed that adults with attention deficit hyperactivity disorder (ADHD) suffer from impairments in a range of cognitive functions when compared to healthy controls. However, only little is known about the neuropsychological functions when compared to various clinical control groups and whether a distinct neuropsychological profile can be identified for adult ADHD. Method This retrospective study examined data of 199 outpatients referred for clinical evaluation of adult ADHD, allocated either to an ADHD group (n = 78) or to one of two clinical comparison groups, depending on whether they show indications (n = 71) or no indications (n = 50) for the presence of psychiatric disorders other than ADHD. All individuals performed a comprehensive neuropsychological test battery. Results Data analysis revealed impairments in a range of cognitive functions in a substantial number of patients of all three groups. However, profiles of neuropsychological impairments were similar between groups. Furthermore, significant small- to medium-sized correlations between basic and higher-order cognitive functions were revealed in the ADHD group and the clinical comparison group with indications for psychiatric disorders other than ADHD. Conclusion Neuropsychological impairments are prominent in psychiatric outpatients seeking a clinical evaluation of adult ADHD but are not specific for ADHD. It is concluded that neuropsychological test performance may have limited incremental value to support the psychiatric differential diagnosis. Furthermore, a clinical trajectory may need to take into account that deficits in a range of higher-order cognitive functions can be substantially explained by deficits in basic cognitive functions.