Of 919 patients with major affective disorders who completed at least 1 year of a 5-year, semiannual follow-up, 45 developed a rapidly cycling bipolar course during the first year, but only one developed a rapidly cycling unipolar course. In comparison with patients who showed a non-rapidly cycling bipolar course, those who became rapid cyclers were more likely to be female and to have exhibited depression, hypomania, or cycling between depression and hypomania within the index episode. Family study data revealed no evidence that high cycle frequencies breed true. Rapid cycling was associated with a significantly lower likelihood of recovery in the second year of follow-up but not in the third, fourth, or fifth. These data suggest that rapid cycling is, in the large majority of cases, a transient, nonfamilial manifestation of bipolar affective disorder.