Objective: To investigate the extent and reasons contributing to discrepancies between those receiving a DSM as against a clinical diagnosis of a bipolar disorder (BP) and/or a borderline personality disorder (BPD). Method: We interviewed participants previously receiving a BP or BPD diagnosis, studying those who met DSM or clinical criteria for one or both conditions. We compared the numbers of participants allocated to the three diagnostic categories according to rater strategy to calculate concordance rates and determine reasons for discordance. Results: Rates of assignment to BP, BPD and comorbid BP/BPD varied according to the diagnostic strategy. Concordance rates were reduced as BP disorder duration criteria were relaxed, with discordance mainly arising from clinical allocation of a BP disorder for those DSM assigned as unipolar depression. Rates of BPD allocation varied marginally, with discordance mostly arising from so clinically diagnosed receiving a comorbid BP/BPD DSM diagnosis. Finally, DSM overestimated comorbidity compared with clinician diagnoses. Of central importance, not imposing the DSM duration criteria for BP did not increase the prevalence of misdiagnosing BPD, a finding at variance with the literature. Conclusion: Rates and reasons for discordance between clinical and DSMdiagnosis are detailed, which should assist clinical decision-making.