This double-blind, placebo-controlled trial was performed to determine the efficacy and tolerability of 8 wk of risperidone augmentation of serotonin reuptake inhibitor (SRI) treatment in adult subjects with treatment-resistant obsessive-compulsive disorder (OCD) (failure of at least two SRI trials). Sixteen adult treatment-resistant OCD patients were randomly assigned to augmentation with 8 wk of either risperidone (n=10) (0.5-3.0 mg/d) or placebo (n=6) following at least 12 wk of SRI treatment. Four patients on risperidone (40%) and none (0%) on placebo were responders with both a Clinical Global Impression - Improvement (CGI-I) score of 1 or 2 and a Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) decrease >/=25%. Risperidone was generally well tolerated: there were 3 dropouts, 1 on risperidone and 2 on placebo. Better Y-BOCS insight score at baseline significantly correlated with a greater CGI-I score at endpoint on risperidone augmentation. Risperidone may be an effective and well-tolerated augmentation strategy in treatment-resistant OCD subjects, but larger sample size studies are required to demonstrate this.