Over the last four years, the Kentucky correctional system has expanded corrections-based modified therapeutic community treatment from 6 programs to 24 programs. To examine the effectiveness of these programs, the state initiated a systematic treatment outcome study known as the Criminal Justice Kentucky Treatment Outcome Study (CJKTOS). The approach involves corrections-based counselors using personal digital assistants (PDAs) to collect and enter clinical assessment data for the baseline. Follow-up interviews 12 months postrelease from incarceration are conducted by a university research team with a stratified random sample of treatment program participants. This article describes the innovative systematic data collection approach for evaluating corrections-based substance abuse treatment currently being used in Kentucky, as well as treatment outcome data for 700 offenders who completed prison or jail-based treatment one year after release using this data collection approach. Findings indicate that the percentage of jail and prison treatment participants reporting any substance use following release from custody was significantly lower at follow-up (43.9%) compared to baseline (94.1%). In addition, there were few differences in substance use at follow-up between offenders who participated in community aftercare compared to those who did not, but those offenders who participated in treatment had reduced rates of recidivism at one-year postrelease. These data suggest positive outcomes from Kentucky corrections-based substance abuse treatment, and provide implications for the importance of a systematic data collection approach to evaluate correctional treatment programs. (Contains 1 figure.)