OBJECTIVE: Duration of untreated illness represents a potentially modifiable component of any diagnosis-treatment pathway. In bipolar disorder (BD), this concept has rarely been systematically defined or not been applied to large clinically representative samples. METHOD: In a well-characterized sample of 501 patients with BD, we estimated the duration of untreated bipolar disorder (DUB: the interval between the first major mood episode and first treatment with a mood stabilizer). Associations between DUB and clinical onset and the temporal sequence of key clinical milestones were examined. RESULTS: The mean DUB was 9.6 years (SD 9.7; median 6). The median DUB for those with a hypomanic onset (14.5 years) exceeded that for depressive (13 years) and manic onset (8 years). Early onset BD cases have the longest DUB (P