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Objective: Recent data from several large studies of pediatric bipolar I disorder reported baseline (current) episode duration ranging from less than a month to ≥1 year. These data may reflect actual sample differences, but the absence of uniformly applied definitions of episode duration, number of lifetime episodes and daily rapid cycling patterns during episodes may also account for these differences.

Method: Proposals for definitions of episode and cycling phenomena were based upon data from the Washington University in St. Louis Kiddie Schedule for Affective Disorders and Schizophrenia (WASH-U-KSADS).

Result: Episode would be used for the interval between onset and offset of full DSM-IV criteria for bipolar I disorder. Cycling would be used only to describe daily (ultradian) switching of mood states that occurs during an episode.

Conclusion: Historically, in the adult bipolar literature the words “episode” and “cycle” were used interchangeably. “Rapid cycling,” in this earlier literature, actually referred to multiple episodes per year. To avoid confusing episodes with daily cycling, the proposal is to use “episode” for the duration of DSM-IV criteria, to use “cycling” for daily switching phenomena during an episode, and to replace the historical term “rapid cycling” with “multiple episodes per year.” These clarifications will be especially important for phenomenological research on preschool populations.