Objective: To investigate the comparative efficacy of aripiprazole and risperidone as augmenting agents in the treatment of obsessive-compulsive disorder (OCD) patients who did not show a ≥35% decrease in the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) after 12-week monotherapy with selective serotonin reuptake inhibitors (SSRIs). Methods: The study consists of two different periods of treatment: a 12-week prospective period to determine resistance to SSRI treatment and an 8-week single-blind addition period for refractory patients only. Ninety patients were randomly assigned to receive one of the SSRI treatments. Sixty-nine patients (76.6%) completed the 12-week SSRI monotherapy period. Forty-one patients (59.4%) were considered refractory and were randomised to receive either risperidone (20 patients, 3 mgr daily) or aripiprazole (21 patients, 15 mgr daily) as augmentation to SSRI treatment. Sixteen patients (76.2%) in the aripiprazole group and 18 patients (84%) in the risperidone group completed the 8-week treatment period. Results: Eight patients (50%) in aripiprazole and 13 patients (72.2%) in risperidone group met response criteria of Y-BOCS decrease ≥35% at the end of the study. The risperidone group showed a significant improvement in Y-BOCS obsession scores compared with aripiprazole. Conclusions The present findings suggest that risperidone may be more effective than aripiprazole. Copyright © 2011 John Wiley & Sons, Ltd.