 Affective instability is one of the most prominent symptoms in borderline personality disorder. Currently there are no prospective studies of treatment of this symptom with mood stabilizing medications. This study examines the effectiveness of lamotrigine compared to placebo in reducing emotional instability of several types in borderline patients. The types of emotional instability studied involve anger, anxiety, depression, and elation. Subjects entering this 12 week study will be blind to whether they are receiving active medication or placebo. They will be asked to report levels of instability in their mood on a weekly basis. They will also be asked weekly to rate the intensity of other symptoms of borderline personality disorder symptoms, such as unstable relationships, self-harm, and impulsive behaviors. 