The co-occurrence of attention deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) has been reported to be highly prevalent in adults. However, very few studies have assessed the usefulness of screening instruments to detect this co-occurrence, particularly when screening for ASD in the context of ADHD. Our study aimed at assessing the utility of the autism-spectrum quotient (AQ) as a screening tool of ASD in a sample of 153 adults referred for ADHD assessment. Our results showed that the AQ is of limited use in this context as its positive predictive value was low (47%). Particularly, the more severe the attentional deficits the more likely individuals with ADHD were to be misclassified as having a co-occurring ASD based on the AQ. However, the "imagination" subscale of the AQ was able to discriminate those who met ASD criteria from those who did not, suggesting that targeting imagination impairments might be useful when assessing for the ADHD+ASD co-occurrence in clinical settings.