Background: Despite the prevalence and clinical significance of borderline personality disorder, its treatment remains understudied. Aims: To evaluate treatment with variably dosed olanzapine in individuals with borderline personality disorder. Method: In this 12-week randomised, double-blind trial, individuals received olanzapine (2.5-20 mg/day; n=155) or placebo (n=159) (trial registry: NCT00091650). The primary efficacy measure was baseline to end-point change on the Zanarini Rating Scale for Borderline Personality Disorder (ZAN-BPD) using last-observation-carried-forward methodology. Results: Both olanzapine and placebo groups showed significant improvements but did not differ in magnitude at end-point (-6.56 v. -6.25, P=0.661). Response rates (50% reduction in ZAN-BPD) were 64.7% with olanzapine and 53.5% with placebo (P=0.062); however, time to response was significantly shorter for olanzapine (P=0.022). weight gain was significantly greater (2.86 v. -0.35kg, P < 0.001), with higher incidence of treatment-emergent abnormal high levels of prolactin for the olanzapine group. Conclusions: Individuals treated with olanzapine and placebo showed significant but not statistically different improvements on overall symptoms of borderline personality disorder. The types of adverse events observed with olanzapine treatment appeared similar to those observed previously in adult populations.