A total of 3674 psychiatric out-patients were evaluated with a semi-structured diagnostic interview for DSM-IV borderline personality disorder (BPD). The affective instability criterion had a sensitivity of 92.8%, higher than the sensitivities of the other eight BPD criteria. The negative predictive value of the affective instability criterion was 99%. We recommend that clinicians screen for BPD in the same way that they screen for other psychiatric disorders: by enquiring about a single feature of the disorder (i.e. affective instability), the presence of which identifies most patients with the disorder and the absence of which rules out the disorder. Copyright and usage (C) The Royal College of Psychiatrists 2017.