It often is difficult clinically to differentiate bipolar disorder from other mental health conditions in young people. This study evaluated a parent report measure of depressive and hypomanic/biphasic symptoms. Parents of 196 youths, who were 5 to 17 years old and presented at an outpatient research center, completed an adapted General Behavior Inventory (GBI). Factor analyses suggested two dimensions, depression (alpha = .97) and biphasic/hypomania (alpha = .95). Logistic regressions using these scales discriminated mood disorder versus disruptive behavior disorder or no diagnosis, unipolar versus bipolar disorder, and bipolar versus disruptive behavior disorder based on structured interviews. Classification rates exceeded 80%, and receiver operating characteristic analyses showed good diagnostic efficiency for the scales, with areas under the curve greater than .80. Results indicate that clinicians can use the parent-completed GBI to derive clinically meaningful information about mood disorders in youths.