The authors review the available literature on the interface between borderline personality disorder and affective disorder. Three competing hypotheses have been offered to explain the substantial overlap between these diagnostic categories; they postulate that borderline disorder arises from affective disorder, that affective disorder arises from borderline disorder, or that the two are independent and overlap coincidentally. None of these hypotheses satisfactorily explains the existing data. The authors propose a fourth hypothesis focusing on the multiple etiologies of the signs and symptoms used to diagnose both affective and borderline disorders and suggesting that some patients in the resulting heterogeneous population have symptom clusters that fit both syndromes.