The present study evaluated the effectiveness of a drug-free unit (DFU) in reducing the use of substances by incarcerated adult offenders, and to analyze changes in the addiction severity index, motivation, and personality caused by the program. This is an external evaluation, with an ex post facto design with repeated measures. Eighty-seven prisoners from the DFU were evaluated during the first year of residence in the program. Most are young men, polydrug addicts and mainly serving sentences for public health crimes and property offenses. There is need of psychiatric treatment at the baseline, with 85% comorbid personality disorders. Motivation for treatment is low, and remains stable over 12 month's duration of the study. The DFU was found to have a significant effect in reducing the use of drugs by offenders and to improve the drug and family composite scores, also reducing scores on personality scales. However, it fails to change medical and psychiatric scores, so that the need for intervention in these areas is underscored.