Objective: Attention-deficit hyperactivity disorder (ADHD) with other psychiatric comorbidities in adulthood is challenging in terms of differentiating the neuropsychological impairments in each disorder. Executive functions have been widely studied and shown to be consistently impaired in ADHD. However, there is a lack of knowledge about which executive function deficits are attributable to ADHD and which of them are assosciated with comorbid psychiatric disorders. Thus, the aim of this study was to investigate to what extent the deficits in executive functions are attributable to ADHD rather than comorbid psychiatric diseases among adult patients with ADHD. Methods: Adult patients diagnosed with ADHD and comorbid psychiatric disorders (n=26) were compared with psychiatric patients without ADHD (n=26) and healthy subjects (n=26) matched for age, sex, and duration of education. Adult ADHD Self-Report Scale, Wender Utah Rating Scale for the ADHD and Structured Clinical Interview for DSM-IV Axis I Disorders screened all participants. In addition, Wisconsin Card Sorting Test computer form, Rey Auditory Verbal Learning Test, and the Stroop Color Word Test were used for neuropsychological assessment. Results: Adults psychiatric patients with ADHD scored lower than those without ADHD in terms of measures of inhibition, attention, planning, set-shifting, problem-solving, verbal learning, and verbal memory. Psychiatric patients without ADHD scored significantly lower than healthy controls in terms of planning and problem solving. Conclusions: Our findings suggest that the ADHD, rather than comorbid psychiatric disorders, is the significant underlying cause for executive function deficits in inhibition, attention, set-shifting, verbal learning, and verbal memory.