A result supports earlier findings of the overall effectiveness of Modified Therapeutic Community (MTC) treatment in correctional settings and encompasses the subgroup of offenders whose co-occurring disorders include Antisocial Personality Disorder (APD). An intent-to-treat analysis and a matched group analysis found significantly greater improvement on measures of criminal activity and substance use for those in the MTC group. For the current population and study conditions, the results suggest that negative treatment effects are not linked to APD, but may point to an association between APD and more positive outcomes. The study provides additional support for the effectiveness of modified therapeutic community (TC) treatment by demonstrating its effectiveness for offenders with APD. Male inmates with MICA disorders were randomly assigned to one of two treatment groups, either MTC or standard mental health services or MH. The MTC program augmented a core of TC principles with treatment modules designed both to meet the needs of the co-occurring disorders population and to change attitudes, behavior, and lifestyles in the three critical areas of substance abuse, mental illness, and criminal thinking/behavior. The MH program provided intensified psychiatric services that consisted of medication, weekly individual therapy and counseling, and specialized groups. Tables, references