A pilot study of a brief group-based Acceptance and Commitment Therapy (ACT) intervention (12 two-hour sessions) was conducted with clients of public mental health services meeting four or more criteria for borderline personality disorder (BPD). Participants were randomly assigned to receive the ACT group intervention in addition to their current treatment (ACT + TAU; N = 21) or to continue with treatment as usual alone (TAU; N = 20). There was significantly more improvement from baseline for the ACT + TAU condition than the TAU condition on the primary outcome variable--self-rated BPD symptoms. The ACT + TAU gain was both clinically and statistically significant. The ACT + TAU condition also had significantly more positive change on anxiety and hopelessness, and on the following ACT consistent process variables: psychological flexibility, emotion regulation skills, mindfulness, and fear of emotions. For all but anxiety, the improvements for the ACT + TAU condition were significant, while the TAU condition had no significant changes on any measure. Follow-up was possible for only a small number of participants. The improvements gained by the ACT + TAU participants were maintained except for fear of emotions. Anxiety continued to improve, becoming significantly different from baseline at follow-up. Examination of mediators found that psychological flexibility, emotion regulation skills and mindfulness, but not less fear of emotions, mediated BPD symptoms. Psychological flexibility and emotion regulation skills also mediated hopelessness. There is a need for a larger trial, for comparison with other established treatments for BPD, and for conducting a trial of a longer intervention. Nonetheless, this pilot study suggests that a brief group-based ACT intervention may be a valuable addition to TAU for people with BPD symptoms in the public sector. (Contains 4 tables, 2 figures and 6 videos.)