The aim was to study the longitudinal course of suicidal behaviour and ideation in patients with borderline personality disorder (BPD) compared with patients with other diagnoses. Ninety-seven patients (41 BPD, 33 other personality disorders, 23 no personality disorder) consecutively admitted to a day unit were given a prospective personal interview follow-up with evaluations at admission, discharge and at follow-up after 2-5 years. Even when controlled for Axis I disorders, BPD patients showed significantly more often a lifetime history of suicide attempts. BPD patients with a history of suicide attempts were more suicidal at index admission, continued to be so over the follow-up period and differed systematically in an unfavourable direction from other BPD patients on the major outcome measures. BPD patients without suicidal behaviour had an outcome nearly as good as non-BPD patients, and only 41% of them retained the BPD diagnosis at follow-up. Suicidal behaviour and ideation are highly prevalent in BPD. These suicidal expressions are of an enduring nature and seem as a diagnostic criterion to enhance the predictive capacity of the BPD diagnosis.