The Level of Supervision (LSI) is a very popular and well-used risk/need assessment tool for assisting Canadian probation and correctional officials to plan and manage the supervision of their charges. LSI has since been adopted for use in numerous jurisdictions in Canada, the United States, and the United Kingdom. It has been in use for the past 17 years, has proven very reliable, and has spawned at least four versions—the latest being the Ontario Revision, or LSI-OR, primarily developed as a case management tool for correctional use. The purpose of this research study was to conduct a longitudinal, predictive validation of the LSI-OR and to examine its ability to predict general and violent recidivism among study samples of prison inmates and probationers; and secondarily to check its predictive validity among exceptional offender groups such as sex offenders, domestic violence offenders, and offenders with mental health problems. The study sample consisted of 630 adult male offenders, of whom 454 were inmates and 176 were probationers. LSI-OR consists of General Risk/Need elements that cover an individual’s history and personal characteristics. These elements are: Criminal History, Education/Employment, Family/Marital, Leisure/Recreation, Companions, Substance Abuse, Pro-criminal Attitude/Orientation, and Antisocial Pattern. LSI-OR also provides for a clinical (discretionary) override in the overall assessment of client risk. Scoring ranges from one (very low risk) to five (very high risk) and the scoring is based on file reviews and interviews of the participants. The results of the validation study indicated that inmates scored higher than probationers in both general and specific risk/need elements of the LSI-OR. The predictive validities of this study of the LSI-OR compare very favorably to those of earlier LSI research and to recent comparative studies. They also support the use of this particular version of the LSI with community (probationers and parolees) and institutional (inmate) offender samples. Although these findings support the use of the LSI-OR with some exceptional offender groups (i.e., sexual, domestic violence, and mental health offenders), caution should be exercised until these results are independently replicated. 8 tables, 3 notes, and 53 references