BACKGROUND Rapid-cycling bipolar disorder is defined as four or more affective episodes yearly. The conventionally recognised limit in episode duration is usually considered 24 hours (i.e. a cycle duration of 48 hours). We report a small series of intensively observed bipolar patients who showed much faster patterns of mood oscillation. METHODS Detailed, systematic, longitudinal assessment of five bipolar patients during extended in-patient psychiatric evaluation were conducted, including retrospective life charting and prospective evaluation of daily mood by self and blinded observer ratings, and motor activity recording. RESULTS Our data demonstrate a spectrum of cycling frequencies in rapid-cyclers, including distinct, clinically robust mood shifts that occur at frequencies faster than once per 24 hours. Affective oscillations spanned a range of cycling frequencies from four episodes per year (rapid cycling) to those occurring within the course of weeks to several days (ultra-rapid cycling), to distinct, abrupt mood shifts of less than 24 hours duration (ultra-ultra rapid or ultradian cycling). The time of onset and duration of these ultradian affective fluctuations are highly variable and they are observed in bipolar patients without evidence of personality disorder. CONCLUSIONS The potential clinical and theoretical implications of these first systematic observations of ultra-rapid and ultradian cycling in the context of the evolution of otherwise classical bipolar affective illness are discussed.