Context: Despite the high morbidity associated with bipolar disorder (BP), few studies have prospectively studied the course of this illness in youth. Objective: To assess the longitudinal course of BP spectrum disorders (BP-I, BP-II, and not otherwise specified [BP-NOS]) in children and adolescents. Design: Subjects were interviewed, on average, every 9 months for an average of 2 years using the Longitudinal Interval Follow-up Evaluation. Setting: Outpatient and inpatient units at 3 university centers. Participants: Two hundred sixty-three children and adolescents (mean age, 13 years) with BP-I (n = 152), BP-II (n = 19), and BP-NOS (n = 92). Main Outcome Measures: Rates of recovery and recurrence, weeks with syndromal or subsyndromal mood symptoms, changes in symptoms and polarity, and predictors of outcome. Results: Approximately 70% of subjects with BP recovered from their index episode, and 50% had at least 1 syndromal recurrence, particularly depressive episodes. Analyses of weekly mood symptoms showed that 60% of the follow-up time, subjects had syndromal or subsyndromal symptoms with numerous changes in symptoms and shifts of polarity, and 3% of the time, psychosis. Twenty percent of BP-II subjects converted to BP-I, and 25% of BP-NOS subjects converted to BP-I or BP-II. Early-onset BP, BP-NOS, long duration of mood symptoms, low socioeconomic status, and psychosis were associated with poorer outcomes and rapid mood changes. Secondary analyses comparing BP-I youths with BP-I adults showed that youths significantly more time symptomatic and had more mixed/cycling episodes, mood symptom changes, and polarity switches. Conclusions: Youths with BP spectrum disorders showed a continuum of BP symptom severity from subsyndromal to full syndromal with frequent mood fluctuations. Results of this study provide preliminary validation for BP-NOS.