A large body of literature has shown a high prevalence of personality disorders in substance abusers. We compared a sample of substance abusers treated in a prison setting with substance abusers treated in a non-prison inpatient setting rated with the Millon Clinical Multiaxial Inventory-III. Base-rate scores indicated a prevalence of 95% of personality disorders. A logistic regression analysis correctly identified 95% of the sample, and showed that antisocial personality disorder traits were characteristic of the prison sample, and masochistic personality disorder traits were characteristic of the inpatient sample. The findings indicate that treatment models used in prison settings should be adjusted to meet the needs of antisocial patients.