We evaluated the efficacy and safety of adjunctive topiramate in bipolar II patients who were either treatment-resistant to or unable to tolerate lithium, carbamazepine or valproate. Nineteen DSM-IV bipolar II patients received increasing doses of open-label topiramate as adjunctive therapy for their hypomanic (n=15) or depressive (n=4) symptoms. Sixteen patients completed the 12-week follow-up. There were highly significant improvements in YMRS, HDRS and CGI-BP-M scores (p=0.0001). Of the fifteen hypomanic patients, eight (53%) were rated as responders to topiramate (50% reduction in YMRS scores), and five (33%) met criteria for remission (YMRS score pound 8). Two of the four patients with a depressive episode at study entry (50%) were rated as responders (50% reduction in HDRS score), and one (25%) achieved remission (HDRS score pound 6). Topiramate was generally well tolerated. One third of the patients experienced weight loss. These preliminary results suggest that adjunctive topiramate may be useful in treating bipolar II disorder.