There is evidence in the literature that youth with intellectual/developmental disabilities (IDD) experience mental health issues to a greater extent compared to youth without IDD. In addition, transition from pediatric to adult services may be a period of increased anxiety and mental health problems for these youth. This study used a retrospective cohort design using population level administrative health data to compare mental health diagnoses between youth with IDD and youth without IDD; particularly examining the period when youth transition from pediatric health services to adult health services. In addition, this study compared the mental health of youth with various types of IDD (Down syndrome, autism, fetal alcohol syndrome and other rarer diagnoses). Results showed that at the population level youth with IDD have greater odds of anxiety/depression or psychotic illness than youth without IDD. Adjusted odds of anxiety/depression or psychotic illness also varied with age, income level, sex, and type of IDD. Youth with fetal alcohol syndrome had the greatest odds of both anxiety/depression and a psychotic illness. Odds of anxiety/depression increased from age 15–19 years old. Odds of a psychotic illness increased from age 15 to age 24 years old.
