Target populations have always been a thorny issue for correctional programs. In this experiment of seamless treatment for probationers in two sites, offenders were randomly assigned to the seamless model (drug treatment incorporated into probation supervision) or traditional referral model to services in the community. The experiment blocked on risk level to measure the differential effects on rearrest and substance abuse. The seamless system model improved treatment participation with greater gains for the offenders who were high risk in both sites. Yet no main effects were observed on drug use or rearrest, although effect sizes illustrate that small effects can be observed for the offenders who are high risk. Part of the failure to observe main effects is because of instrumentation problems, namely, that most substance abusers in the experiment had low-severity substance abuse problems and were marijuana users. The focus on sound dynamic factors may assist with identifying the appropriate target populations for correctional interventions. (Contains 9 tables.)