A large body of studies demonstrates mild cognitive dysfunction in patients with Obsessive Compulsive Disorder (OCD). Few trials have investigated whether this dysfunction can be improved by treatment. Thirty unmedicated inpatients with OCD were administered a comprehensive neuropsychological test battery before and after 12 weeks of cognitive behavioral therapy (CBT). Thirty-nine carefully matched healthy controls were tested twice within the same interval. At baseline, patients exhibited significant impairments on several tests which normalized at follow-up. A significant group x time interaction was found for tests of nonverbal memory, set shifting and flexible, self guided behavior. Major responders improved significantly more than minor responders on the Rey-Osterrieth Figure immediate and delayed recall. Results suggest that cognitive dysfunction in OCD can improve in the course of treatment. We hypothesize that particularly cognitive behavioral treatment enables OCD patients to think and act in a more flexible way that helps them to develop more effective cognitive strategies.