The relationship between psychopathy and antisocial personality disorder (APD) was explored in a sample of Australian mentally disordered offenders. Two Hare adult psychopathy measures, the Psychopathy Checklist-Revised (PCL-R) and the Psychopathy Checklist: Screening Version (PCL: SV) were employed and a diagnosis of APD was measured for 136 participants in a secure forensic psychiatric inpatient facility. Results revealed clear distinctions between measurements of psychopathy and APD. Over 65% of patients high in psychopathic traits received a diagnosis of APD while only 5.5% of patients with APD were high in psychopathic traits, denoting an asymmetric relationship. Implications for the assessment and treatment of mentally disordered offenders with psychopathic traits are discussed.