Autism spectrum disorders (ASD) often co-occur with intellectual disability (ID) and are associated with poorer psychosocial and family-related outcomes than ID alone. The present study examined the prevalence, stability, and characteristics of ASD estimates in 2,208 children with ASD and ID identified through the South Carolina Autism and Developmental Disabilities Network. The prevalence of ASD in ID was 18.04%, relative to ASD rates of 0.60%–1.11% reported in the general South Carolina population. Compared to children with ASD alone, those with comorbid ID exhibited increased symptom severity and distinct DSM-IV-TR profiles. Further work is needed to determine whether current screening, diagnostic, and treatment practices adequately address the unique needs of children and families affected by comorbid ASD and ID diagnoses.