OBJECTIVE: To assess lamotrigine effectiveness in bipolar disorder (BD) patients in a clinical setting. METHOD: Open lamotrigine was naturalistically administered to outpatients at the Stanford University BD Clinic assessed with the Systematic Treatment Enhancement Program for BD (STEP-BD) Affective Disorders Evaluation, and monitored longitudinally with the STEP-BD Clinical Monitoring Form. RESULTS: One hundred and ninety-seven patients (64 BD I, 110 BD II, 21 BD NOS, 2 Schizoaffective Bipolar Type, mean+/-SD age 42.2+/-14.4 years, 62% female) had 200 trials of lamotrigine. Lamotrigine was combined with a mean of 2.1+/-1.5 other psychotropic medications, most often during euthymia or depressive symptoms. Mean lamotrigine duration was 434+/-444 days, and mean final dose was 236+/-132mg/day without valproate, and 169+/-137mg/day with valproate. Lamotrigine was discontinued in only 26.5% of trials at 255+/-242 days, most often due to inefficacy, and seldom due to adverse effects. In 31.5% of trials lamotrigine was continued 264+/-375 days with no subsequent psychotropic added. In 42.0% of trials lamotrigine was continued 674+/-479 days, but had subsequent psychotropic added at 146+/-150 days, most often for anxiety/insomnia and depressive symptoms. In 145 trials started at Stanford, lamotrigine primarily yielded relief of depressive symptoms or maintained euthymia. In 55 trials in which lamotrigine was started prior to Stanford, lamotrigine primarily maintained euthymia. Lamotrigine was generally well tolerated, with no serious rash, and only 3.5% discontinuing due to benign rash. CONCLUSION: In a cohort of bipolar disorder outpatients commonly with comorbid conditions, and most often receiving complex combination therapy, lamotrigine had a low (26.5%, with an overall mean duration of treatment of 434 days) discontinuation rate, suggesting effectiveness in BD in a clinical setting.