Patients with personality disorders who did not respond to previous outpatient treatment are among the most challenging patients to treat and are often referred to specialized settings. Acceptance and commitment therapy (ACT) is an innovative therapy that has shown effectiveness in treatment-resistant cases with chronic or recurrent depression with or without co-morbid personality disorders. The central role that ACT accords to positive values and experiential avoidance may enhance treatment responsivity in patients with personality disorders that did not respond to previous treatments. The current nonrandomized study explored the effectiveness of a 26-week ACT-based group treatment (n = 60) for personality disorders compared to treatment-as-usual (n = 21) based on cognitive behaviour therapy (CBT-TAU) at a specialized setting for patients with personality disorders. Individuals in both treatment conditions demonstrated small to moderate decreases in general psychological functioning and personality pathology. There was no main effect of therapy condition. Overall, results suggest that ACT is a possible treatment option for individuals with difficult-to-treat personality pathology and further outcome research is warranted.