Individuals with Alcohol Related Neurodevelopmental Disorder (ARND) and Attention-Deficit/Hyperactivity Disorder (ADHD) display similar deficits in behavioural, cognitive, and executive dysfunction symptoms; however, the underlying impairment in brain function and attention pathways is thought to be different. This study compared these two clinical groups, and healthy controls, using psychological assessments and functional magnetic resonance imaging (fMRI). The two clinical groups had significantly different scores on measures of overall intellectual functioning, working memory, and the conjunction trials on the fMRI assessment but could not be differentiated on other measures from rating scales, standardized psychological assessments, and performance data from fMRI tasks. The fMRI task accuracy variables were strongly correlated with related standardized psychological measures. All groups demonstrated difficulties with response inhibition compared to attention, and the clinical groups demonstrated more difficulties with attention and variability compared to the control group on a computer-paced Go/No-Go task. The comparison of a self-paced and a computer-paced Go/No-Go task indicated that the computer-paced task would be more appropriate to use with fMRI to assess cortical activation in response inhibition. It was found that the ADHD group had higher levels of cortical activation (indicating that more cognitive effort was require to reach the same level of behavioural performance) compared to the ARND group during the visual-spatial attention tasks, whereas the ARND group had higher levels of activation during the response inhibition and working memory tasks. Despite no significant differences in behavioural performance, the fMRI tasks helped to demonstrate different activation patterns that could help distinguish and differentially diagnose these two similar groups. / October 2016