Four subtypes of bipolar disorder (BP) - bipolar I, bipolar II, cyclothymia and bipolar not otherwise specified (NOS) - are defined in DSM-IV-TR. Though the diagnostic criteria for each subtype are intended for both adults and children, research investigators and clinicians often stray from the DSM when diagnosing pediatric bipolar disorder (PBD) (Youngstrom, 2009), resulting in a lack of agreement and understanding regarding the PBD subtypes.The present study uses the diagnostic validation method first proposed by Robins and Guze (1970) to systematically evaluate cyclothymic disorder as a distinct diagnostic subtype of BP. Using a youth (ages 5-17) outpatient clinical sample (n=827), participants with cyclothymic disorder (n=52) were compared to participants with other BP spectrum disorders and to participants with non-bipolar disorders.Results indicate that cyclothymic disorder shares many characteristics with other bipolar subtypes, supporting its inclusion on the bipolar spectrum. Additionally, cyclothymia could be reliably differentiated from non-mood disorders based on irritability, sleep disturbance, age of symptom onset, comorbid diagnoses, and family history.There is little supporting research on cyclothymia in young people; these analyses may be considered exploratory. Gaps in this and other studies are highlighted as areas in need of additional research.Cyclothymic disorder has serious implications for those affected. Though it is rarely diagnosed currently, it can be reliably differentiated from other disorders in young people. Failing to accurately diagnose cyclothymia, and other subthreshold forms of bipolar disorder, contributes to a significant delay in appropriate treatment and may have serious prognostic implications.