Background: Paediatric obsessive-compulsive disorder (OCD) is highly comorbid with other psychological disorders, including attention deficit/hyperactivity disorder (ADHD). Preliminary evidence suggests that youth with comorbid OCD and ADHD may experience greater impairments than children with other comorbidities; however, there is limited research examining the clinical expression and treatment response of these youth. Methods: Youth (7 to 17 years) with a primary diagnosis of OCD and comorbid ADHD (n = 40) were compared a sample of age and gender matched youth with OCD and other comorbidity (without ADHD, n = 40). The study investigated symptoms, severity, functioning, comorbidity, family accommodation, in addition to parental psychopathology and rearing styles. Treatment response was investigated at post-treatment and six-month follow-up. Results: Youth with comorbid OCD and ADHD had fewer sexual obsessions, higher rates of comorbidity, poorer executive functioning and higher family impairment. Families of comorbid youth engaged in significantly more accommodation and reported more negative rearing. Finally, comorbid youth were significantly less likely to be responders or remitters at post-treatment. Limitations and Conclusions: Limitations include the cross-sectional design, relatively small clinical sample, and lack of an experimental control group of youth with ADHD without OCD. Current approaches to treatment may be improved for youth with comorbid OCD and ADHD by addressing cooccurring anxiety, behavioural difficulties, and maladaptive family accommodation and rearing. Moreover, given pronounced deficits in executive function, these youth may require a stronger initial dose of CBT to achieve an adequate response.