This is the largest population-based study of ADHD-related symptoms in young children with ASD in association with cognitive and behavioral scores. Our results confirm the high prevalence of attention deficits in children aged 2–5 with ASD, relative to children with typical development or developmental delay. We found significant associations between higher levels of attention deficits in children with ASD and lower scores on cognitive and behavioral assessments. These results suggest ADHD symptoms in young children with ASD play an important role in impacting other areas of functioning.