Abstract
Borderline personality disorder (BPD) and bipolar disorder (BD) could present a diagnostic challenge in the adolescent due to the presence of overlapping symptoms such as impulsivity, affective instability, and sexual arousal. Of these symptoms, affective instability is a central feature of BPD, and there is a rapid shift from the neutral affect to an intense affect, and this is associated with a dysfunctional modulation of emotions. We describe three Sri Lankan adolescents presenting with affective instability, treated with psychopharmacological agents as for BD. While BPD is characterized by transient mood shifts induced by interpersonal stressors, in BD, there are sustained mood changes. A longitudinal assessment of the symptomatic profile and collateral information clarified the diagnosis as being BPD. An examination of the nature of affective instability is vital for a proper diagnosis and provision of evidence-based treatment.