Many studies have compared the demographic and clinical characteristics of patients with and without borderline personality disorder (BPD), but there is limited knowledge on differences within the population of borderline patients. One potential index of heterogeneity is disorder severity. In the present report from the Rhode Island Methods to Improve Diagnostic Assessment and Services (MIDAS) project, we examined whether the severity of borderline personality disorder, as measured by the number of criteria present, is associated with co-morbidity of Axis I and Axis II diagnoses, as well as demographic factors and psychosocial functioning. Two thousand three hundred psychiatric outpatients were interviewed with the Structured Interview for DSM-IV Personality (SIDP-IV). Approximately ten percent (n = 237) of the patients were diagnosed with BPD, and they were divided into four groups based on the number of DSM-IV criteria met, 5 (n = 89), 6 (n = 70), 7 (n = 46), and 8 or 9 (n = 32). There were greater rates of drug use disorders and comorbid Axis II disorders, as well as a greater number of suicidal gestures, in patients meeting seven or more BPD criteria. There were no significant differences between the groups in the number and specific rates of other co-morbid Axis I disorders, other measures of psychosocial functioning, or demographic correlates. Sub-typing of borderline patients by the number of criteria met provides a limited explanation for heterogeneity within BPD patients.