Patients with BPD are characterized by a pervasive pattern of instability in affect regulation, impulse control, interpersonal relationships, and self-image. The efficacy of fluvoxamine and lamotrigine for affective instability in patients with BPD has been shown in controlled studies. However, no study has evaluated the effectiveness and safety of lamotrigine for treating irritability in adolescent patients with BPD in Japan. Here, we report the case of a patient with BPD whose condition improved after lamotrigine administration. Lamotrigine could be effective and safe for treating BPD-associated affective instability. However, large-scale controlled studies are needed to confirm our findings.