The prevalence of autism spectrum disorder (ASD) has increased in recent decades, much of which is related to changes in diagnostic criteria, and greater awareness among professionals and parents. Using a prospective cross-sectional study design, this study explores the prevalence of ASD among 173 adolescents admitted to two psychiatric facilities in Canada, and its association with some early pre and perinatal risk factors. The overall prevalence of ASD in the psychiatric population was 11.56% compared to 1.52% in children and youth in Canada. While prenatal and perinatal factors were not significantly associated with ASD, we found a frequent association of ASD with different comorbid psychiatric conditions. These findings further our knowledge in planning and management of ASD among this population.