To investigate the relationship of sadistic personality disorder (SPD), as defined in the appendix of DSM-III-R, to other personality disorders and to sexual sadism, 70 sex offenders (27 child molesters, 33 rapists, and 10 murderers) were assessed by the International Personality Disorder Examination. In 19 subjects (27.2%) from the total sample, SPD was diagnosed. The highest overlap appeared with borderline personality disorder (31.6%) and antisocial personality disorder (42.1%). However, in four cases SPD was the only personality disorder diagnosed. Factor analysis of the antisocial and sadistic criteria resulted in four major factors--one factor with high loadings on the sadistic criteria and the violent criteria of antisocial personality disorder, two factors with different forms of adult and juvenile aggression, and a fourth factor with high loadings on the antisocial criteria covering exploitative behavior. The results do not support SPD as a discrete disorder. Nevertheless, SPD may be seen as an important subdimension of antisocial personality disorder, distinct from more exploitative forms of antisocial behavior with less violence. Of those patients with SPD, 42.1% also had a DSM-III-R diagnosis of sexual sadism, which may be the most dangerous configuration.