Objectives: The aim of this study was to examine whether personality i.e. temperament and character interacts with age of onset in bipolar disorder. Methods: Bipolar patients were recruited among in- and outpatients from lithium dispensaries of northern Sweden. Patients were diagnosed according to DSM-IV criteria for bipolar disorder type I and II. Temperament and Character Inventory (TCI) was used for measuring personality. TCI was administered to 100 lithium treated bipolar patients and 100 controls. Results: Treatment response was significantly lower (p = 0.005) in patients with early onset compared with late onset. Family history (p = 0.013) and suicide attempts (p = 0.001) were also significantly more common in patients with early onset. Further, patients with early onset were significantly higher (p = 0.045) in the temperament factor harm avoidance (HA) than patients with late onset, but the difference was weak. Patients with early onset had more fear of uncertainty (HA2, P = 0.022) and were more shy (HA3, p = 0.030). Bipolar I patients showed similar results as those in the total bipolar group (I and II), with significantly higher HA (p = 0.019, moderate difference), HA2 (p = 0.015) and HA3 (p = 0.043) in patients with early onset compared with late onset. Bipolar II patients showed no differences between early and late age of onset but the groups are small and the results are therefore uncertain. Conclusions: Early age of onset in bipolar disorder was correlated to an increase in severity, family history, poorer treatment response and poorer prognosis. Early onset was also correlated to personality.