Background: Previous studies about the effectiveness of Dialectical Behaviour Therapy for the treatment of Borderline Personality Disorder have had promising results. However, no previous studies have examined its effectiveness when delivered in low secure inpatient services for women. Aims: To evaluate clinical outcomes during and after a 1-year period of admission within a low secure unit for women offering a Dialectical Behaviour Therapy programme. Method: A naturalistic, within subjects study of clinical data collected as part of routine practice was conducted. Participants were18 consecutively admitted women who met the diagnostic criteria for Borderline Personality Disorder and had completed at least 1 year of treatment. Measures covered: risk behaviours; self-reported symptoms of Borderline Personality Disorder, and current mood and symptom experience; staff reports of clinical problems, needs and social functioning. Scores were compared between admission and at 6 months and 1 year. Results: There was a statistically significant improvement on all 13 measures over the year's treatment. Most improvement was demonstrated between admission and 6 months. Conclusions: Engagement in1-year's treatment was associated with significant reduction in risk behaviours and both staff-rated and self-rated outcome measures. Some significant questions remain about which elements of the programme are most effective but the results are encouraging.