After screening and classification, treatment is provided in groups of 8 to 12 inmates, using a cognitive- behavioral, skills-based approach that includes a focus on relapse prevention. Program goals are to encourage long-term abstinence through prevention of lapse and relapse to substance abuse and to involve participants in ongoing treatment services following release from jail. An untreated control group was established to assist in the comparison of post-treatment outcomes for treated and untreated offenders. Attempts were made to contact all persons who had been released from the program for 6 months or more, so as to assess changes in psychosocial functioning that occurred during the follow-up period. The evaluation findings indicate that over a 1-year period following release from custody, inmates who participated in the 6-week jail treatment program remained significantly longer in the community until rearrest, experienced fewer arrests, and served less jail time compared to the group of untreated inmates. Significant improvements were also found in program participants' abilities to use coping skills to respond effectively to high-risk situations for drug relapse, in acquisition of knowledge regarding relapse prevention and recovery principles, and in self-confidence in dealing with high-risk situations. Implications are drawn for the development of jail substance abuse treatment programs and for additional research.