Different issues pertaining to the study of Rapid Cycling Affective Disorders (RCAD) are examined. RCAD's appear to be relatively infrequent, even though their clinical characterization does not differ from the conventional affective disorders. RC can be classified as early onset and late onset, according to time of appearance; spontaneous and induced (by pharmacological or nonpharmacological agents), according to causation; and ultra-rapid (48 hours) and classical (three days to 12 weeks), according to cycle length. Case reports seem to show that the ultra-rapid cyclers may have clear differences when compared with the classical rapid cyclers: elderly male patients seem to start their affective picture with RC, whereas younger female patients are predominantly classical rapid cyclers. The inducers seem to be far less specific and the response to treatment far less consistent in the ultra-rapid cyclers. The most widely accepted etiopathogenic theory ascribes an important role to thyroid hypofunction, probably precipitated by lithium treatment. The review, however, suggests that this theory may explain only a minority of cases. Treatment approaches remain tentative, inconsistent and even conflicting with the proposed etiopathogeneses. Clearly, terminological precisions, well defined clinical samples, neurohormonal and psychophysiological tests, and epidemiological designs, can and should help elucidate the key question of whether RCAD is an autonomous clinical condition or simply one end in the spectrum of cyclicity. © 1985.