The study first made a retrospective analysis of documented disciplinary problems and general personal information in a large California prison, where drug treatment has existed for over 4 years. The program uses a traditional drug treatment therapeutic community model and is operated by a private nonprofit corporation that supplies the counseling staff. The inmates in the treatment unit participate in some activities with inmates in the general prison population. Fourteen indicators were used to examine adverse inmate behavior. The findings show a clear and statistically significant tendency toward much less problematic inmate behavior in the treatment yard compared to general custody yards. The number of Classification A Reports (the worst possible category for inmate adverse behavior) were significantly fewer in the treatment yard than in the rest of the institution. The number of Classification D Reports, reflecting physical violence by inmates, in the treatment yard were more than 10 times fewer than in non-treatment yards. The number of incident reports in the treatment yard was less than one-fourth of that in general custody yards. Further, there was a dramatic reduction in occupational injuries in staff non-related to assault in the treatment yard compared with the rest of the institution. Translated into personal safety and professional comfort, these findings show a significantly less stressful work environment for correctional officers assigned to the treatment yard, as well as for the inmates in treatment. 14 figures and 6 references