In the present study, we examined how unstructured (e.g., spending time with friends or co-workers) and structured (e.g., attending social events at a place of workshop, sports teams) social participation changed from before to after high school for youth with autism spectrum disorders (ASD), as well as the longitudinal and concurrent relations between social participation and internalizing symptoms. Participants included 36 families of youth with ASD who were all in their last year of high school at the first time point of data collection, and who were out of high school for an average of 9 months at the second time point. Social participation and internalizing symptoms were determined using parental report. There was no average change in the amount of unstructured social participation after high school exit, although substantial individual variability was observed. Participation in structured social activities significantly declined after high school exit. Youth who had more structured social participation while in high school were significantly more likely to have gains in their unstructured social participation after high school exit. Turning to relationships between internalizing and social activities, more internalizing symptoms while youth with ASD were in high school significantly predicted increasing social isolation after high school exit (both in terms of structured and unstructured activities). Results point to the likely need for additional supports during the transition to adulthood for youth with ASD who have internalizing problems. Autism Res2016. (c) 2016 International Society for Autism Research, Wiley Periodicals, Inc. Autism Res 2017, 10: 663-672. (c) 2016 International Society for Autism Research, Wiley Periodicals, Inc.