This study examined the interdiagnostician reliability and potential gender bias of the DSM-IV/DSM-5 Section II and DSM-5 Alternative Model definitions of borderline personality disorder. A national sample of 123 mental health professionals provided diagnostic judgments on 12 case vignettes selected to represent a range of personality pathology. Two versions of each case were included, one identified as male and the other as female, but which were otherwise identical. Analyses examined the intraclass correlation between clinicians and also examined rates of diagnostic assignments as a function of case gender. Reliability of diagnosis of borderline personality did not differ across the two diagnostic approaches, and concordance of diagnoses across the two systems was significant. The dimensional components of the DSM-5 Alternative Model demonstrated significantly more diagnostic reliability than the DSM-IV categorical diagnoses. The DSM-5 Alternative Model conceptualization of borderline personality can be diagnosed with comparable or greater reliability than the extant DSM-IV definition.