BACKGROUND: The UK guidelines on the treatment of personality disorder recommend avoiding compulsory treatment except in extreme situations. Little is known about how often patients with personality disorder are detained or how this compares with the treatment of other mental disorders. OBJECTIVES: Our aim is to test the hypothesis that people with personality disorder are infrequently detained under the Mental Health Act (MHA) and that risk factors associated with detention are the same as those for people with other mental disorders. METHOD: We used a retrospective, quantitative study of MHA assessments. RESULTS: Of the 2 087 assessments undertaken, 204 (9.8%) patients had a diagnosis of personality disorder; 40.7% of assessments in the personality disorder group resulted in detention, as did 69.7% of patients with other mental disorders. A higher proportion of people with personality disorder received no intervention following assessment compared with those with other mental disorders (20.6% vs. 4.7%, p < 0.001). Study centre and a history of admission were risk factors for detention in both groups. Risk was a predictor of detention in those with other mental disorders. CONCLUSIONS: Detention rates in patients with personality disorder are lower than those for other disorders but are still substantial. Risk factors for detention in patients with personality disorder differ from those with other mental disorders. Copyright © 2016 John Wiley & Sons, Ltd.