In recent years, growing evidence supports the efficacy of antipsychotic addition to serotonin reuptake inhibitors in patients with treatment-refractory obsessive-compulsive disorder. This study is the first to investigate the effects of antipsychotic addition on cognitive functioning in obsessive-compulsive disorder patients. Cognitive functioning was evaluated at baseline and at endpoint of an 8-week double-blind, placebo-controlled quetiapine addition trial. Neurocognitive performance was compared between the placebo and quetiapine group and between responders and nonresponders. The neuropsychological test battery consisted of the national adult reading test, the Wisconsin Card Sorting Test, the Trail Making Test, verbal fluency, the Stroop Color Word Test, the California Verbal Learning Test, the Rey Complex Figure Task, the Continuous Performance Test, and the Digit Symbol Substitution. The results of this study suggest that quetiapine addition to serotonin reuptake inhibitors has no major effects on cognitive functioning in obsessive-compulsive disorder patients, apart from some evidence for a failure to maintain set on the Wisconsin Card Sorting Test. It is proposed that this failure may be caused by attention difficulties owing to somnolence.