Background: No instrumnt exists that can predict the incidence of bipolar disorders (BD). The Bipolarity index (BI), originally developed to improve diagnostic confidence for a lifetime diagnosis of BD, may predict incident BD. Aim: To assess the predictive performance of the BI for incident BD in persons with a lifetime depression. Methods: The BI score was composed from different questionnaires and interviews in n = 1857 subjects without BD and with a lifetime unipolar depressive disorder from the Netherlands Study of Depression and Anxiety, a longitudinal cohortstudy. The incidence of DSM-IV defined BD I or II as a criterion diagnosis was established with the Composite International Diagnostic Interview after 2, 4, 6 and 9 years of follow-up. Cox regression analyses calculated whether the BI predicts incident BD during 9-years of follow-up. The area Under the Curve (AUC) was determined. At the optimal cut-off score, sensitivity, specificity, positive, and negative predictive values (PPV and NPV) were calculated. Results: Over the course of 9 years, bipolar conversion occurred in n = 46 subjects (2.5%). Each point increase in BI score significantly predicted incident BD (HR[95%CI]= 1.047[1.018–1.076], p = 0.001). The AUC was 0.61 (95%CI: 0.54–0.68). At the optimal cut-off of 30, sensitivity was 67%, specificity 52%, PPV 3% and NPV 98%. Limitations: Not all items of the BI were fully covered; mean age of the sample of 42. Conclusion: The BI score predicts bipolar conversion over 9 years in those with a lifetime depression. However, given the modest performance metrics, the BI cannot guide clinical decision making yet.