Introduction: Findings regarding the relationship between IQ and comorbidity in autism and ADHD are inconsistent. Methods: Mothers rated 1,436 children with autism and 1,056 with ADHD without autism (IQs 9-149, ages 2-17) on the Pediatric Behavior Scale. Results: Children with IQs < 70 (vs. [greater than or equal to] 70) had less oppositional behavior, irritability, tantrums, conduct problems, generalized anxiety, and depression and more separation anxiety. Children with ADHD-Inattentive had less comorbidity than children with autism and children with ADHD-Combined. For autism, 80% had ADHD-Combined symptoms and 9% had ADHD-Inattentive symptoms. Most children with autism and with ADHD-Combined with IQs [greater than or equal to] 70 had oppositional defiant disorder, as did almost half with IQs < 70. For autism, 45% with IQs [greater than or equal to] 70 had generalized anxiety and 47% with IQs < 70 had separation anxiety. Conclusions: Given high rates of comorbid symptoms in autism and ADHD, clinicians should be alert to and assess for psychiatric comorbidity.