Sixty-four percent of the sample had a drug-use history, and this group was more likely to be rearrested and to fail more quickly while on probation compared with probationers without a drug-use history. Seventy-one percent of the probationers with a drug-use history received treatment, and most completed treatment. As hypothesized, probationers who completed the full course of drug treatment were the least likely to reoffend among those with a drug-use history; however, the reoffending rate of drug-treatment completers was similar to that of probationers who had no drug-use history. Probationers who entered but did not complete drug treatment were more likely to reoffend than drug users who needed treatment but did not receive it. The results suggest that drug treatment can reduce reoffending only if participants complete the full course of treatment. Treatment programs that include an employment component or facilitate gainful employment may increase the chances of treatment completion and reduce opportunities for reoffending. Further, the value of keeping a probationer in treatment may outweigh the value of terminating program participation due to technical probation violations. The study sample included all probationers discharged from supervision in Illinois from October 30 through November 30, 2000. Data were collected through a questionnaire administered by probation officers in each county. It obtained information on probationer demographic characteristics, offender behavior while on probation, sentencing and case outcomes, and reoffending. Information on drug use and treatment, criminal histories, and probation outcomes was obtained from official court and probation records. Arrest data were obtained from State criminal justice records in order to determine reoffending in the 4 years following probation discharge. 4 tables, 2 figures, 43 references, and appended description of variables