The program is a therapeutic community of selected inmates having substance abuse problems. Clients are housed in a unit separate from the general prison population. The male and female programs have operated for over 8 years. Parole outcome information was obtained for 1,626 male and 398 female participants. The treatment groups included all clients who had completed the program through February 28, 1984. The program reduced recidivism, and the time spent in the program was positively related to increases in time until arrest for those who did recidivate. Maximum treatment benefit was achieved by clients in treatment 9 to 12 months. Clients who remained more than 12 months showed some reduction in treatment benefit. The evaluation recommends ample support for such programs and the establishment of a strong re-entry phase after 1 year of in-prison treatment. 3 tables, 3 figures, 23 references.