Bipolar disorder (BD) is a debilitating illness that often starts at an early age. Prevention of first and subsequent mood episodes, which are usually preceded by a period characterized by subthreshold symptoms is important. We compared demographic and clinical characteristics including severity and duration of subsyndromal symptoms across adolescents with three different bipolar-spectrum disorders. Syndromal and subsyndromal psychopathology were assessed in adolescent inpatients (age = 12-18 years) with a clinical mood disorder diagnosis. Assessments included the validated Bipolar Prodrome Symptom Interview and Scale-Prospective (BPSS-P). We compared phenomenology across patients with a research consensus conference-confirmed DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition) diagnoses of BD-I, BD-not otherwise specified (NOS), or mood disorder (MD) NOS. Seventy-six adolescents (age = 15.6 ± 1.4 years, females = 59.2%) were included (BD-I = 24; BD-NOS = 29; MD-NOS = 23) in this study. Median baseline global assessment of functioning scale score was 21 (interquartile range = 17-40; between-group  = 0.31). Comorbidity was frequent, and similar across groups, including disruptive behavior disorders (55.5%,  = 0.27), anxiety disorders (40.8%,  = 0.98), and personality disorder traits (25.0%,  = 0.21). Mania symptoms (most frequent: irritability = 93.4%,  = 0.82) and depressive symptoms (most frequent: depressed mood = 81.6%,  = 0.14) were common in all three BD-spectrum groups. Manic and depressive symptoms were more severe in both BD-I and BD-NOS versus MD-NOS ( < 0.0001). Median duration of subthreshold manic symptoms was shorter in MD-NOS versus BD-NOS (11.7 vs. 20.4 weeks,  = 0.002) and substantial in both groups. The most used psychotropics upon discharge were antipsychotics (65.8%; BD-I = 79.2%; BD-NOS = 62.1%; MD-NOS = 56.5%,  = 0.227), followed by mood stabilizers (43.4%; BD-I = 66.7%; BD-NOS = 31.0%; MD-NOS = 34.8%,  = 0.02) and antidepressants (19.7%; BD-I = 20.8%; BD-NOS = 10.3%; MD-NOS = 30.4%). Youth with BD-I, BD-NOS, and MD-NOS experience considerable symptomatology and are functionally impaired, with few differences observed in psychiatric comorbidity and clinical severity. Moreover, youth with BD-NOS and MD-NOS undergo a period with subthreshold manic symptoms, enabling identification and, possibly, preventive intervention of those at risk for developing BD or other affective episodes requiring hospitalization.