The purpose of this study was to identify diagnostic and severity subgroups within a cohort of patients with borderline personality disorder (BPD). Of 171 patients clinically diagnosed with BPD, 132 were Diagnostic Interview for Borderlines (DIB) positive. Through a process of random selection, 41 of the DIB positive subjects were also interviewed with the revised version of the DIB (DIBR) and approximately one half with two semi-structured research interviews, the Schedule for Affective Disorders (SADS) and the Personality Disorder Examination (PDE). All subjects completed four self-report measures of problematic behaviours, symptoms and social adaptation. The analyses included examination of: 1. the correspondence of the BPD diagnosis across the DIB, the DIBR and the PDE; 2. the association between DIBR scoring levels and scores on measures of symptoms and behavioural status; and 3. the co-occurrence of BPD with Axis I and other Axis II disorders. Correlations and analyses of variance between both the DIB and DIBR scoring levels and the scores on the four symptom and behavioural measures showed that the scoring levels (DIB 7 to 10; DIBR 8 to 10) could be used to distinguish three subgroups of BPD. The three groups differed in terms of concordance for BPD with the PDE and in terms of patterns of overlap with DSM-III-R, Axis I and other Axis II disorders. This study shows that BPD subgroups can be located on a continuum of symptomatic and behavioural severity and that each subgroup has a specific pattern of overlap with Axis I and other Axis II disorders.