ObjectiveThe epidemiological data on the diagnostic course of patients with borderline personality disorder (BPD) is limited. We used a nationwide register to investigate the diagnostic stability and changes over time. MethodThe Danish nationwide registers were used to follow all patients with a diagnosis of BPD and investigate their first-ever psychiatric diagnosis and their latest diagnosis in the time period of 1995-2012. From this, we found the diagnostic stability and described the diagnostic changes. ResultsA total of 10786 patients diagnosed with BPD were identified. The prospective diagnostic stabilities were 37% for females and 25% for males, and retrospective stabilities were 20% for females and 22% for males. More than 60% of patients received other diagnoses than BPD as their first-ever diagnosis. Stress-related (17%) and depressive disorders (14%) were most frequent as first-ever diagnosis. The latest diagnosis remained BPD in nearly half of females and one third of males, followed by schizophrenia, notably for those with longer follow-up and males. ConclusionThis study gives a detailed display of complicated clinical trajectories. The low diagnostic stabilities demonstrate a heterogenous patient group diagnosed with many other psychiatric diagnoses over time. Copyright (c) 2016 John Wiley & Sons, Ltd.