This study aimed to characterise the neuropsychological functioning of adults with comorbid attention deficit hyperactivity disorder (ADHD) and intellectual disability. Individuals with ADHD and mild-borderline range intelligence (N=59) and individuals with ADHD and normal intellectual functioning (N=95) were compared on attentional and response inhibition tasks. The comorbid group had significantly lower scores on the majority of measures in comparison with the ADHD alone group. These differences remained significant after co-varying for level of intellectual functioning for variables measuring selective attention and errors of commission during sustained attention. This suggests that individuals with comorbid ADHD and intellectual disability may be vulnerable to a 'double deficit' from both disorders in certain aspects of cognitive functioning.