Background <p>This study investigated the associations among treatment motivation, patient adherence, and treatment outcomes in a group of outpatients with difficult-to-treat obsessive–compulsive disorder (OCD).</p> Methods <p>A total of 163 relapsed or nonresponding patients were treated with the Bergen 4-day treatment. Motivation was measured with a modified version of the Nijmegen Motivational List 2 (NML2) prior to the start of treatment. During treatment, patients rated their own adherence using the Patient Exposure and Response Prevention (EX/RP) Adherence Scale (PEAS). Treatment outcomes were assessed with the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) at posttreatment and 3-month follow-up.</p> Results <p>The NML2 items related to commitment showed a weak but significant association with treatment outcomes but were not significant when patient adherence was controlled for. Higher adherence was strongly associated with better treatment outcomes.</p> Discussion <p>Self-reported motivation before the start of treatment had limited predictive validity, whereas in- and between-session patient adherence, which could reflect one aspect of the motivation construct (treatment engagement), was important for recovery.</p> Trial registration <p>ClinicalTrials.gov identifier: NCT02656342 (First registered: 2015–11–30).</p>