Lamotrigine is an anticonvulsant that appears to have a mainly antidepressant effect and is indicated for the maintenance treatment of bipolar depression. Literature associated with obsessional symptoms related to lamotrigine treatment is limited. We report the emergence of obsessive symptoms during treatment with lamotrigine in a patient who subsequently experienced significant improvement after dose reduction and stopping of this medication. The obsessive symptoms associated with lamotrigine treatment were observed after the lamotrigine dose was increased to 100 mg/day. The possible mechanisms, including inhibition on the presynaptic release of glutamate and alteration of striatal dopamine uptake, are discussed. It is unclear why lamotrigine induces obsessions in some patients. Controlled studies are necessary to identify the population at risk for obsessionality in bipolar illness following treatment with lamotrigine and to investigate a possible dose-response relationship between obsessive symptoms and lamotrigine.