Cognitive behavioural therapy (CBT) is the first-line psychological treatment for Obsessive-Compulsive Disorder (OCD). However, 30% of individuals have a null or partial response. Preliminary evidence suggested that Acceptance & Commitment Therapy (ACT) may be effective. No study investigated whether the association of CBT with ACT may improve outcomes of CBT alone.

This paper presents the protocol of a trial where individuals with OCD will be randomly assigned to CBT alone or CBT plus ACT. Primary endpoints will be the number of individuals meeting OCD diagnostic criteria at post-treatment and follow-up. Secondary endpoints will be self-reported depression, anxiety, disgust and guilt, and obsessive beliefs. It is hypothesized that CBT plus ACT is associated to fewer individuals meeting OCD criteria and greater reductions in secondary endpoints.

A single-blinded superiority randomised design will be used. Primary/secondary outcomes will be administered at baseline, post-treatment and 6-month follow-up. Treatment duration will be 25 weekly sessions in both conditions. Individuals (age ≥ 18 years) with OCD diagnosis will be recruited at mental health services in a 60.000 inhabitants area in Italy. Chi squared will be computed to test group differences on OCD diagnosis. ANCOVAs will be calculated entering baseline scores as covariates, group allocation as random factor and primary/secondary outcomes as dependent variables.

To obtain a medium effect size, 80% power and 0.05 significance, a priori power analysis suggests inclusion for at least 34 individuals as total sample.

A description of the protocol will be provided. Strengths and potential limitations will be addressed.