- Backgrond Autism spectrum disorder (ASD) symptoms and attention deficit/ hyperactivity disorder (ADHD) symptoms are frequently comorbid with obsessive-compulsive disorder (OCD). However, limited research exists with respect to the relations between these symptoms, and their impact on OC symptom severity. Method 109 outpatients with primary OCD and 87 healthy controls were administered OCD, ADHD, and ASD questionnaires. Univariate analyses, correlations, and stepwise regression analyses were conducted. Results OCD, ADHD, and autism symptoms were highly correlated, with OCD patients showing elevated ADHD as well as autism scores when compared with healthy controls. OCD patients with comorbid ADHD presented with higher autism symptoms and OCD symptoms, yet similar OCD severity scores as OCD without ADHD patients. The attention switching and lack of social skills subscales of the Autism Questionnaire (AQ) were particularly correlated with ADHD and OCD symptoms in the patient group. The AQ subscale attention switching proved to be the most significant predictor of OCD severity and symptoms (with the exception of hoarding). Contrary to expectations, the AQ subscale attention to detail did not predict OCD symptoms, nor did any of the AQ subscale scores predict hoarding symptoms. Conclusions OCD patients present with elevated scores of ADHD and ASD symptoms, and these symptoms (particularly attention switching) are important in predicting OC symptoms and severity. It is suggested that problems in attention may be related to the uncertainty about one's own memory as demonstrated by OCD patients, as well as a common factor underlying comorbid ADHD and ASD symptoms.