The most effective treatment programs for addicted offenders have been intensive programs of considerable duration that are designed as modified therapeutic communities. The treatment principles of the Cornerstone Program include: (1) clearly understood rules and consequences; (2) formal participation by inmates in running the program; (3) treating addiction and criminality; and (4) transition and aftercare. Results of this study show that: (1) the Cornerstone Program continues to demonstrate a positive effect on decreasing the criminal activity of program participants; (2) addicted offenders who receive little or no treatment show an accelerating pattern of criminal activity; and (3) time in treatment correlates positive with success and that many of these treated offenders continue to show some involvement with the criminal justice system after treatment. Finally, results suggest that arrests, convictions, or incarcerations are all approximately equally accurate measures of criminal activity. 3 figures, 2 tables, and 6 references (Author abstract modified)