In order to clarify differential-diagnostic questions 47 mentally ill sex offenders (with impulse control disorder and paraphilia) were assessed with regard to comorbidity of personality disorders and "psychopathy". For this examination the SKID-II for personality disorders and the Psychopathy Checklist (PCL-R) were used. 72 % of the sex offenders showed at least one personality disorder. The highest prevalence was found for cluster-B disorders, first of all the antisocial personality disorder. Using a PCL-R cut off score of 25 (for Europe) 10 of the 47 subjects (21 %) were diagnosed as "psychopaths", whereas being a "psychopath" is associated with a number of different personality disorders. The importance of structured diagnostic with regard to comorbidity of personality disorders in sex offenders imprisoned in maximum security psychiatric hospitals is stressed, and furthermore there is evidence that the PCL-R is a valid instrument for prognosis, but not a psychological-psychiatric instrument for the assessment of personality disorders.