The aim of this study is to test the feasibility and acceptability of an internet-delivered cognitive behavior therapy (ICBT) intervention originally developed for neurotypical adolescents with obsessive compulsive disorder (OCD) in adolescents with OCD and autism spectrum disorder (ASD). This feasibility study will inform us how to adapt the original intervention to the specific needs of patients with ASD.

To evaluate the effectiveness of ICBT for OCD in adolescents with ASD, this study will use a single case experimental design with weekly measures during two phases: A-B. The A-phase will consist of baseline measures at least three weeks prior to treatment. The B-phase will consist of the 12 weeks ICBT-program as well as one measure post-treatment. Single case descriptions of each participant will be presented. Weekly measures on the Obsessional Compulsive Inventory - Child version (OCI-CV) will be analyzed through visual inspection of each participants graph and through computation of Percentage of Non-overlapping Data (PND). PND is a frequently used effect measure in SCED and reflects how large percentage of the measurements during the treatment-phase that do not overlap with any of the measurements during the base-line phase.

To assess the feasibility and comprehension of the ICBT treatment semi-structured interviews (face-to-face or telephone) will be conducted with participants and parents weekly during the four first weeks of ICBT and semi-weekly during the rest of the treatment. Detailed notes will be taken during each interview and will serve as content material for a thematic analysis trying to determine possible accommodations to the ICBT protocol to better suit patients with ASD.

Inclusion Criteria:

• a diagnosis of OCD as defined by DSM-5 [40]
• a diagnosis of autism defined by DSM-5
• a total score of ≥ 16 on the Children's Yale-Brown Obsessive-Compulsive Scale, CY-BOCS [14]
• age between 12 and 17 years
• ability to read and write Swedish
• access to the internet
• a parent that is able to co-participate in the treatment
• Participants on psychotropic medication must have been on a stable dose for the last 6 weeks prior to baseline assessment

Exclusion Criteria:

• psychosis or bipolar disorder, severe eating disorder
• suicidal ideation
• subject not able to read or understand the basics of the ICBT material
• completed CBT for OCD within last 12 months (defined as at least 5 sessions of CBT including exposure and response prevention)
• ongoing psychological treatment for OCD or another anxiety disorder