The intriguing nature of rapid cycling in some affective disorder patients raises many differential diagnostic questions. On the basis of a clinical case study, the suggestion is made that rapid cycling affective disorder (RCAD) patients may present manifestations resembling a multiple personality disorder (MPD) because of a premorbid borderline personality. The clinical similarities between RCAD and borderline personality disorder (BPD) are pointed out and emphasis is placed on the role of the borderline's instability, anger, regression, and use of dissociative mechanisms to induce pseudomultiplicity. The existence of MPD as an autonomous clinical entity is examined. The therapeutic relevance of this differential diagnostic approach is also emphasized.