Although difficulty with affect regulation is generally considered a core component of borderline personality disorder (BPD), surprisingly little research has focused on the nature of affect regulation and dysregulation in BPD. A random national sample of 117 experienced clinicians provided data on a randomly selected patient with BPD (N = 90) or dysthymic disorder (DD; N = 27). Clinicians described their patients using the Affect Regulation and Experience Q-sort-Questionnaire Version, a psychometric instrument designed for expert informants to assess affect and affect regulation. BPD and DD patients appear to differ in both the emotions they experience and the ways they regulate or fail to regulate them. Whereas DD patients are characterized by negative affect, BPD patients are characterized by both negative affect and affect dysregulation, which appear to be distinct constructs. BPD patients also show distinct patterns of affect regulation, and subtypes of BPD patients show distinct affect regulation profiles of potential relevance to treatment.