Background: This study examined whether a combination of the Awareness and Care for my Autistic Traits (ACAT) program and treatment-as-usual (TAU) is more effective than only TAU in increasing the understanding of autistic attributes, reducing treatment stigma, and improving mental health and social adaptation among adolescents with autism spectrum disorder (ASD) and their parents/guardians. Methods: Forty-nine adolescents and their parents/guardians were randomly assigned to either a combination of ACAT and TAU (COMB) group or only TAU group. The COMB group received six weekly 100-min ACAT sessions while the TAU group received no additional intervention. The primary outcome was the change in the understanding of autistic attributes (Autism Knowledge Quiz-Child: AKQ-C), administered to autistic adolescents from pre-to post-intervention. The secondary outcomes included the change in AKQ-parent version (AKQ-P) administered to their parents/guardians, treatment stigma, improving mental health, and social adaptation among autistic adolescents and their parents/guardians. All measures except for the self-rating scales were scored by asessors blind to the group assignment. Results: The individuals (both autistic adolescents and their parents/guardians) in the COMB group, showed an increased score of the AKQ-C and AKQ-P than did those in the TAU group. Autistic adolescents in the COMB group also demonstrated reduced treatment-related stigma and behavioral problems than those in the TAU group, while there were no group differences in the change in social adaptation. For parents/guardians, there were no group differences in the change in treatment-related stigma, mental health, adaptive skills, or attitudes toward their children. Conclusions: The results suggest that the ACAT could be an effective treatment modality for both autistic adolescents and their parents/guardians to increase the understanding of autistic attributes. The ACAT may positively affect self-understanding, reduce treatment stigma, and stabilize behavioral issues for autistic adolescents, but it may not effectively reduce treatment barriers or improve mental health for parents/guardians. Further research should consider whether additional support for parents/guardians could be beneficial. Trial registration: Registration number - UMIN000029851 and date of registration - 06/01/2018.
