Objectives: This paper reviews the literature to examine the DSM-IV diagnostic criteria for rapid cycling in bipolar disorder. Methods: Studies on the clinical characteristics of rapid cycling bipolar disorder were reviewed. To identify relevant papers, literature searches using PubMed and MEDLINE were undertaken. Results: First observed in the prepharmacologic era, rapid cycling subsequently has been associated with a relatively poor response to pharmacologic treatment. Rapid cycling can be conceptualized as either a high frequency of episodes of any polarity or as a temporal sequence of episodes of opposite polarity. The DSM-IV defines rapid cycling as a course specifier, signifying at least four episodes of major depression, mania, mixed mania, or hypomania in the past year, occurring in any combination or order. It is estimated that rapid cycling is present in about 12-24% of patients at specialized mood disorder clinics. However, apart from episode frequency, studies over the past 30 years have been unable to determine clinical characteristics that define patients with rapid cycling as a specific subgroup. Furthermore, rapid cycling is a transient phenomenon in many patients. Conclusions: While a dimensional approach to episode frequency as a continuum between the extremes of no cycling and continuous cycling may be more appropriate and provide a framework to include ultra-rapid and ultradian cycling, the evidence does not exist today to refine the DSM-IV definition in a less arbitrary manner. Continued use of the DSM-IV definition also enables comparisons between past and future studies, and it should be included in the next release of the ICD. Further scientific investigation into rapid cycling is needed. In addition to improving the diagnostic criteria, insight into neurophysiologic mechanisms of mood switching and episode frequency may have important implications for clinical care.