Abstract Background: Pediatric bipolar (BP)-I disorder affects a sizeable minority of children and is associated with high levels of morbidity. Relatively few studies have assessed the persistence of the disorder over time. Objective: The main aim of this study was to extend our findings from our 4-year follow-up study examining rates of persistence of pediatric BP-I disorder onto late adolescent years and young adulthood 5 years after our original study. Methods: We conducted a 1-year extension to our original prospective study of 78 youth, ages six to 17 years, with BP-I disorder at ascertainment, who were followed up into their adolescent and young adult years (14.9 ± 3.8). All subjects were comprehensively assessed with structured diagnostic interviews and psychosocial, educational, and treatment history assessments. Results: Of the 78 BP-I participating youth, 68 were re-accessioned one year following the 4-year follow-up study, thus effectively 5 years since the original study. Of these, 63% continued to meet full (50%) or subthreshold (13%) diagnostic criteria for BP-I and 18% continued to have full or subthreshold major depressive disorder. Only 19% of BP-I youth were euthymic at the 5-year follow up. Discussion: This 1-year extension study further documents the high level of persistence of pediatric BP-I from childhood onto late adolescence and young adulthood. The results provide compelling evidence of the morbidity and dysfunction associated with this disorder and its many forms. Clinical significance: This study adds to a small amount of literature on the persistence of pediatric BP disorder and the critical need for early identification and intervention.