Affective instability is, as Dr Murdock notes, a fundamental component of borderline personality disorder. However, although there exists a rich literature concerning the links and boundaries between borderline personality disorder and the affective disorders, we know of no references concerning overlaps between borderline personality disorder and rapid cycling per se. Moreover, with the exception of antisocial personality disorder, our study did not assign categorical personality disorder diagnoses and we are forced to approach this question indirectly. It is relevant here that rapid cycling was almost entirely a bipolar phenomenon, while the mood lability typical of individuals with borderline personality disorder involves, according to DSM-III-R criteria, "marked shifts from baseline mood to depression, irritability or anxiety...." Also, Loranger et al1 and others have noted high rates of major depression, but not of bipolar disorder, among the relatives of probands with borderline personality disorder. Clinical experience tells us that frequent shifts