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 < 0.0001). An extended DUB was associated with more mood episodes (P < 0.0001), more suicidal behaviour (P = 0.0003) and a trend towards greater lifetime mood instability (e.g. rapid cycling, possible antidepressant‐induced mania). Conclusion:  Duration of untreated bipolar disorder (DUB) will only be significantly reduced by more aggressive case finding strategies. Reliable diagnosis (especially for BD‐II) and/or instigation of recommended treatments is currently delayed by insufficient awareness of the early, polymorphous presentations of BD, lack of systematic screening and/or failure to follow established guidelines.