BACKGROUND: Borderline personality disorder (BPD) is a severe and chronic psychiatric disorder characterized by marked impulsivity, instability of affect and interpersonal relationships, and suicidal behavior that can complicate medical care. Few data are available on its prevalence or clinical presentation outside of specialty mental health care settings. METHODS: We examined data from a survey conducted on a systematic sample (N = 218) from an urban primary care practice to study the prevalence, clinical features, comorbidity, associated impairment, and rate of treatment of BPD. Psychiatric assessments were conducted by mental health professionals using structured clinical interviews. RESULTS: Lifetime prevalence of BPD was 6.4% (14/218 patients). The BPD group had a high rate of current suicidal ideation (3 patients [21.4%]), bipolar disorder (3 [21.4%]), and major depressive (5 [35.7%]) and anxiety (8 [57.1%]) disorders. Half of the BPD patients reported not receiving mental health treatment in the past year and nearly as many (6 [42.9%]) were not recognized by their primary care physicians as having an ongoing emotional or mental health problem. CONCLUSIONS: The prevalence of BPD in primary care is high, about 4-fold higher than that found in general community studies. Despite availability of various pharmacological and psychological interventions that are helpful in treating symptoms of BPD, and despite the association of this disorder with suicidal ideation, comorbid psychiatric disorders, and functional impairment, BPD is largely unrecognized and untreated. These findings are also important for the primary care physician, because unrecognized BPD may underlie difficult patient-physician relationships and complicate medical treatment.