Objective: To identify the early signs of mood disorder development, specifically bipolar disorder (BD), in a population at familial risk for BD.Method: The sample included 107 Dutch adolescent bipolar offspring (age 12-21) followed into adulthood (age 22-32). Lifetime DSM-IV axis I diagnoses were examined at baseline, 1-, 5-, and 12-year follow-up. Symptoms were assessed at baseline on a 3-point Likert scale at baseline with the K-SADS-PL and were analyzed using symptom and sum scores. As observed in previous studies, BD typically starts with other mood disorders. Therefore, the sample was stratified in offspring with a mood diagnosis (n = 29) and without (n = 78) at baseline.Results: Subthreshold manic experiences proved the strongest predictor of BD conversion (n = 10; HR2.16, CI95% 1.23-3.78). At symptom level, elated mood, decreased need of sleep, racing thoughts, suicidal ideation, and middle insomnia were significantly associated with BD conversion. Depressive symptoms proved the strongest predictor for first mood episode onset (n = 28; HR1.27, CI95% 1.02-1.58).Conclusion: This study extends our knowledge of prodromal manifestations of BD in a high-risk population. Although preliminary, findings of this study provide potential targets for early identification and underscore the importance of detailed assessment of manic symptomatology in bipolar offspring.