DSM-IV-TR defines four subtypes of bipolar disorder (BP): bipolar I, bipolar II, cyclothymic disorder and bipolar not otherwise specified (NOS). However, cyclothymic disorder in children is rarely researched, or often subsumed in an "NOS" category. The present study tests the replicability of findings from an earlier study, and expands on the criterion validity of cyclothymic disorder in youth. Using the Robins and Guze (1970) framework we examined the validity of cyclothymic disorder as a subtype of BP. Using a youth (ages 5-17) outpatient clinical sample (N = 894), participants with cyclothymic disorder (n = 53) were compared to participants with other BP spectrum disorders (n = 399) and to participants with non-bipolar disorders (n = 442). Analyses tested differences in youth with cyclothymic disorder and bipolar disorder not otherwise specified who do, and those who do not, have a parent with BP. Compared to youth with non-bipolar disorders, youth with cyclothymic disorder had higher irritability (p less than 0.001), more comorbidity (p less than 0.001), greater sleep disturbance (p less than 0.005), and were more likely to have a family history of BP (p less than 0.001). Cyclothymic disorder was associated with a younger age of onset compared to depression (p less than 0.001) and bipolar II (p = 0.05). Parental BP status was not significantly associated with any variables. Results support that cyclothymic disorder belongs on the bipolar spectrum. Epidemiological studies indicate that cyclothymic disorder is not uncommon and involves significant impairment. Failing to differentiate between cyclothymic disorder and bipolar NOS limits our knowledge about a significant proportion of cases of bipolarity.