Background: Previous studies have found a strong association between dissociation and obsessive-compulsive disorder (OCD). The purpose of the present study was to evaluate whether dissociation is a predictor of cognitive behavior therapy (CBT) outcome in patients with OCD. Methods: Fifty-two patients with OCD were assessed using the Dissociative Experience Scale (DES), the Yale-Brown Obsessive-Compulsive Scale and the Beck Depression Inventory. CBT lasted on average 9.5 weeks and included exposure therapy. Results: Patients who dropped out due to noncompliance had higher baseline DES scores and depression scores compared to the 43 patients (83%) who completed the study. Significant OCD symptom reduction at posttreatment was observed in study completers with a large effect size (d = 1.7). More severe OCD symptoms at posttreatment were associated with higher DES scores at baseline, and treatment nonresponders had significantly higher baseline DES scores compared to responders. These associations with outcome were mainly due to the DES subfactor absorption-imaginative involvement. In regression analyses, higher absorption-imaginative involvement scores at baseline predicted poorer CBT outcome, even after controlling for depressive symptoms, comorbid axis I disorders and concomitant psychotropic drugs. Conclusions: Results from this preliminary study suggest that higher levels of dissociation (particularly absorption-imaginative involvement) in patients with OCD might predict poorer CBT outcome. If our results can be replicated, treatment outcome might be improved by additional interventions for those patients with OCD who indicate high levels of dissociation, for example by using interventions aimed at improving coping with emotionally stressful situations.