Data were collected from pre-release and 12-month follow-up interviews with drug-using offenders. The prison-based drug treatment program consisted of three phases: adjustment, psychological counseling, and social adjustment. Each phase was about 1 month in duration. At the end of each phase, each participant was evaluated based on his/her performance on each treatment component plus overall performance on daily activities. No systematic evaluation of program outcomes has been conducted since the implementation of the compulsory prison drug treatment program. A quasi-experimental design was employed consisting of experimental and control groups. It was found that Taiwan’s prison-based drug treatment program did not have positive effects with respect to criminal recidivism, drug relapse, increasing social and family functioning, and improving mental health among drug-using offenders. These results are inconsistent with findings presented in the Western literature, and suggest that the prison-based treatment program in Taiwan is correlated with criminal recidivism and post-release drug use. Compared to offenders that received no treatment, offenders that got the drug treatment were more likely to engage in criminal recidivism and drug relapse. They were more likely to report having problems in adjusting to community life after release, especially in the areas of finance, drug abuse, family, and law. They were also more likely to report problems related to poor control of anger and suicidal thoughts. The data linked several post-treatment characteristics to a greater likelihood of criminal recidivism and drug relapse: number of friends and immediate family members using illicit drugs, living arrangement after release, and maladjustment to post-release life. 7 tables, 48 references