Proponents of effective correctional treatment services have found it challenging to demonstrate how the principles of effective treatment can be put into place effectively in the real world, specifically prisons. This study had three goals: (1) to evaluate the overall effectiveness of a prison-based treatment to reduce recidivism; (2) to examine the influence of the “dosage” on treatment effectiveness and recidivism; and (3) to evaluate the effectiveness of staff to assign offenders to varying “dosages” of treatment. The study sample consisted of 620 offenders, who had been released for a minimum of 1 year from the Rideau Correctional and Treatment Center (RCTC), a medium security facility in Ontario, Canada. All the study participants were assessed for risk to reoffend and screened for criminogenic needs. In this study, the primary determinants of recommended length of treatment were the offender’s risk to reoffend and how many criminogenic needs he had. The results indicated that staff recommended offenders to different lengths of treatment consistent with the principles of risk, need, and responsivity. The study showed how to practically apply the principles of effective correctional treatment inside a prison, that treatment worked, and that “dosage” or length of treatment played a significant role in reducing recidivism. Tables, references