Background: Rapid cycling, defined as 4 or more mood episodes per year, is a course specifier of bipolar disorder associated with relative treatment resistance. Several risk factors have been suggested to be associated with rapid cycling. The purpose of this meta-analysis was to compare clinical studies for the evidence of discriminating factors between rapid and non-rapid cycling. Data Sources and Selection: We searched MEDLINE and reference lists of articles and book chapters and selected all of the clinical studies published from 1974 to 2002 comparing subjects with rapid and non-rapid cycling bipolar disorder. Prevalence rates and mean random effect sizes for 18 potential risk factors that were reported by at least 3 studies were calculated. In addition, we differentiated between current and lifetime diagnoses of rapid cycling. Data Synthesis: Twenty studies were identified. Rapid cycling was present in 16.3% of 2054 bipolar patients in 8 studies that included patients who were consecutively admitted to an inpatient or outpatient facility, without a priori selection of rapid cyclers and without matching the numbers of rapid cyclers to non-rapid cycling controls. Female gender and bipolar II subtype both had a small, but statistically significant, effect (p < .000 for female gender, p < .001 for bipolar II sub-type). The further absence of recurrences with lithium prophylaxis was reported in 34% of rapid cyclers compared with 47% of non-rapid cyclers, a nearly significant difference, and a partial response was present in 59% and 65% of patients, respectively. The effect of hypothyroidism was significant (p < .01) in studies using current, but not lifetime, definitions of rapid cycling. In 46% of cases, a rapid cycling course was preceded by treatment with antidepressants, but systematic data on their causal role are lacking. Conclusion: Rapid cycling is slightly more prevalent in women and in patients with bipolar II subtype. In contrast to common opinion, lithium prophylaxis has at least partial efficacy in a considerable number of rapid cyclers, especially when antidepressants are avoided. Hypothyroidism may be associated with mood destabilization in vulnerable patients.