Purpose: The co-occurrence of autism spectrum disorder (ASD) and attention- deficit/hyperactivity disorder (ADHD) is significant and associated with a host of negative outcomes. Studies investigating social functioning in the presence of the ASD/ADHD co-occurrence have produced mixed findings. The present study further evaluated the impact of co-occurring ADHD on social functioning among youth with ASD and compared treatment response to a social competence intervention between youth with ASD and ASD + ADHD. Methods: Two-way repeated measures analyses of variance (ANOVAs) were computed with diagnostic group and time as the independent variables and measures of social functioning as dependent variables. Group and Time effects and Group by Time interactions were examined. Results: Youth with co-occurring ADHD displayed more impairments related to social awareness, but not in other social areas. Participants in both the ASD and ASD + ADHD groups demonstrated significant improvement following a social competence intervention. Conclusion: Co-occurring ADHD did not negatively affect treatment response. Youth with ASD + ADHD may benefit highly structured interventions with a scaffolded teaching design.