summary A 45-year-old patient with a medication-resistant obsessive-compulsive disorder was treated successfully by topiramate augmentation. Obsessive-compulsive disorder is associated neurobiologically with overactivation of the cortico-striato-thalamo-cortical circuit. Because the neurotransmitter glutamate plays an important role in this circuit, medication that has glutamate antagonism, such as the antiepileptic drug topiramate, may reduce the symptoms of obsessivecompulsive disorder. Further study is needed to find out whether topiramate augmentation is more effective than placebos in patients with a medication-resistant obsessive-compulsive disorder.