Background.Deep brain stimulation (DBS) is a promising new treatment for patients with treatment-refractory obsessive–compulsive disorder (OCD). However, since most DBS patients only show a partial response, the treatment still needs to be improved. In this study we hypothesized that cognitive–behavioural therapy (CBT) could optimize the post-operative management in DBS and we evaluated the efficacy of CBT as augmentation to DBS targeted at the nucleus accumbens.Method.A total of 16 patients with treatment-refractory OCD were treated with DBS targeted at the nucleus accumbens. After stabilization of decline in OCD symptoms, a standardized 24-week CBT treatment programme was added to DBS in an open-phase trial of 8 months. Changes in obsessive–compulsive, anxiety and depressive symptoms were evaluated using the Yale–Brown Obsessive Compulsive Scale, Hamilton Anxiety Scale and Hamilton Rating Scale for Depression.Results.Following the addition of CBT to DBS, a significant decrease in obsessive–compulsive symptoms was observed, but not in anxiety and depressive symptoms. In a subsequent double-blind phase, in which stimulation was discontinued, OCD symptoms returned to baseline (relapse) and anxiety and depressive symptoms worsened (rebound) compared with baseline.Conclusions.The results of this explorative study suggest that a combined treatment of accumbens DBS and CBT may be optimal for improving obsessive–compulsive symptoms in treatment-refractory OCD. However, a subsequent randomized controlled trial is necessary to draw firm conclusions. It seems that DBS results in affective changes that may be required to enable response prevention in CBT. This may indicate that DBS and CBT act as two complementary treatments.