While correctional institutions are generally hostile environments that impede attempts at both treatment and research, both can be accomplished successfully, even though it is highly difficult to maintain the integrity of treatment programs and research studies within correctional facilities. This study reports treatment findings for the Stay'n Out therapeutic community (TC), which has operated in the New York State correctional system for over 12 years. This is the first large-scale study (N = 1,500) that provides convincing evidence that prison-based TC treatment can produce significant reductions in recidivism rates for males and females. 1 figure, 5 tables, 27 references. (Publisher abstract)