The research was conducted at the Amity prison therapeutic community (TC) located at the R. J. Donovan Correctional Facility, a medium-security facility in San Diego. The program separates treatment participants from the general inmate population in a 200-bed housing unit. The program has three phases: an orientation, the assuming of increased responsibility for one's behavior and program participation, and prerelease preparation for release into the community. The therapeutic regimen involves encounter groups and individual counseling. Inmates who complete the prison TC, which lasts 9 to 12 months, can volunteer to enter Vista aftercare upon release. This is a community-based TC operated by Amity. Parolees can remain in Vista for up to 1 year. The program evaluation used an experimental design with a randomized assignment of clients into a no-treatment control group and an intent-to-treat group (inclusive of all participants assigned to treatment condition). A total of 715 inmates participated in the study. Data were collected for variables related to preadmission, TC treatment, community TC aftercare treatment, and postrelease follow-up. The study focused on background information and 3-year reincarceration data maintained by the California Department of Corrections. Complete data were obtained on 679 subjects (278 controls and 401 intent-to-treat). The findings clearly replicated the 1999 findings of Knight et al., which suggest that high-risk inmates benefited more than low-risk inmates from prison treatment. Low-risk participants in prison treatment had 3-year reincarceration rates that were nearly the same as those of low-risk participants without treatment; however, this general finding was attenuated when treatment subgroup was included in the analysis. Large reductions in recidivism were found for both high-risk and low-risk participants. Treatment effects were also significant for both high-risk and low-risk groups when aftercare was added to in-prison TC treatment. Overall, the results support providing in-prison TC plus aftercare for both high-risk and low-risk inmates. Suggestions are offered for future research. 6 tables and 23 references