Objective: According to DSM-IV, bipolar disorders (BDs) include four subtypes, BD I, BD II, cyclothymic disorder, and BD not otherwise specified (NOS). We explore the clinical implications of this subtyping in a naturalistic sample of referred youths with BD I, BD II, and BD-NOS. Method: The sample consisted of 217 patients, 135 males and 82 females, ages between 8 and 18 years (mean age, 13.6 [+ or -] 2.9 years), diagnosed according to historical information, prolonged observations, and a structured clinical interview (Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime version). The location of the study was the Stella Maris Scientific Institute of Child Neurology and Psychiatry of Pisa (Italy). Results: Seventy-eight patients (35.9%) had BD I, 97 (44.7%) had BD II,and 42 (19.4%) had BD-NOS. Patients with BD I presented more frequently psychotic symptoms and elated rather than irritable mood. Patients with BD II were less severely impaired, presented more frequently depression as the intake affective episode, and had the highest comorbidity with anxiety disorders. Patients with BD-NOS presented an earlier onset of the disorder, a chronic rather than episodic course, an irritable rather than an elated mood, and a more frequent comorbidity with attention-deficit/hyperactivity disorder and oppositional defiant disorder. Conclusions: DSM-IV categorization of BD may have meaningful implications in youths, but needs to be detailed further. (Contains 3 tables.)