The use of biologic markers in the evaluation of borderline personality disorder (BPD) patients is reviewed. Many patients with Axis II BPD have coexisting Axis I diagnoses of which depression is the most commonly reported. Biologic markers have not aided in the diagnosis of BPD, but some markers, particularly EEG sleep, are not only abnormal in BPD, but also appear to discriminate Axis I depression from other Axis I codiagnoses. Monoamine oxidase, in vitro red blood cell lithium ratio, and P300 auditory evoked potential when used alone or in a combined diagnostic approach, show promise in identifying these codiagnoses as well. Dexamethasone suppression and thyrotropin-releasing hormone tests appear nonspecific in this population. Pharmacologic trials have demonstrated that some BPD patients have good therapeutic response to antipsychotics and tranylcypromine and poor response to alprazolam.