This study evaluated the effectiveness of low intensity behavioral treatment (on average 6.5h per week) supplementing preschool services in 3-6-year-old children with autism spectrum disorder and severe to mild intellectual disability. Treatment was implemented in preschools (i.e., daycare centers) and a discrete trial teaching approach was used. Twelve children in the treatment group were compared to 22 children receiving regular intervention. At pre-treatment, both groups did not differ on chronological age, developmental age, diagnosis and level of adaptive skills. Eight months into treatment, children receiving behavioral treatment displayed significantly higher developmental ages and made more gains in adaptive skills than children from the control group. No significant differences between groups were found on autistic symptom severity and emotional and behavioral problems. (Contains 2 tables.)