ISP participants attend meetings with ISP officers, group counseling sessions, and specialized treatment meetings. Officers and counselors attempt to convince the ISP participant to adopt socially acceptable goals and reject negative behaviors. Activities aim to promote self- discipline. Deterrence is implemented through close supervision designed to detect any offense or rule violation. Every detected rule violation carries some punishment. The primary goal of this evaluation was to determine the degree to which deterrence/rational-choice variables were effective in reducing the rate of drug-use recidivism. The research cohort consisted of the 546 incarcerated offenders who were released into ISP between January 1, 1989, and April 30, 1990. Data were obtained on key variables early in the participant's exposure to ISP, before most of the drug-use recidivism occurred and then again after either drug use had reoccurred or the participant had successfully refrained from drug use for at least a year in ISP. Drug-use follow-up interviews were conducted with 159 participants. The evaluation concluded that strict supervision with a credible threat of substantial punishment deterred at least 10 percent and possibly as many as 60 percent of drug users from resuming drug use for at least a year. The researchers are reluctant to claim that 60 percent was due exclusively to deterrence, since ISP provides rehabilitative strategies as well. Tabular data, 65 references, and appended baseline interview schedule of questions