Borderline personality disorder is common across mental health settings and associated with substantial suffering, disability, and mortality risk. However, it is often underdiagnosed and no prevalence data are available for state hospital settings. This study aims to provide a preliminary assessment of the prevalence and characteristics of borderline personality disorder (BPD) within an inpatient unit of a state psychiatric hospital. In order to improve the quality of care at a state psychiatric hospital, all patients admitted to a 40-bed unit from April 2008 to June 2009 were routinely administered a structured diagnostic interview for BPD. A total of 65 patients were screened and their charts were reviewed. Twenty patients (31%) met criteria for BPD, but only eight of them (40%) had a chart diagnosis of BPD. Compared to patients without BPD, patients with BPD were significantly more likely to be female (75%), to carry a diagnosis of major depressive disorder, and to have been admitted within the past year. Twenty-four percent of patients with a primary psychotic disorder also met criteria for BPD. BPD patients tended to have shorter lengths of stay, but they had significantly more management problems, including incidents of self-harm, episodes of restraint, and stat administrations of medications. The authors conclude that BPD can be a common but underappreciated co-occurring condition in a state psychiatric hospital. In this setting, BPD is associated with more frequent admissions and numerous management challenges, suggesting the need for comprehensive screening, coordination of care, and specialized treatment programming.