While more historical analyses of sex offender treatment programs determined that sex offender recidivism could not be markedly improved through treatment, more recent research has been optimistic about the possibility of reducing sexual re-offenses through treatment. The current study probed the effectiveness of sex offender treatment through a longitudinal analysis of California’s SOTEP program; the study involved the first randomized trial of a modern cognitive-behavioral approach with incarcerated adult sex offenders. The effectiveness of SOTEP was analyzed by comparing the post-release activities of 3 matched groups of offenders: (1) a relapse prevention group of 259 inmates who volunteered to participate and were randomly assigned to the SOTEP program; (2) another group of 225 sex offenders who volunteered to participate and were randomly assigned to no treatment; and (3) a third group of 220 inmates who qualified for the program but refused to participate. Following treatment and release, participants were followed in the community for 5 to 14 years; data included Federal Bureau of Investigation records, California Department of Corrections reports on criminal activity, parole violations, and re-arrests, as well as phallometric assessments of deviant sexual interests. Results indicated no statistically significant differences between the three groups on their rates of sexual or violent re-offending over an 8-year follow-up period. The lack of significant differences held true for both rapists and child molesters. While the results do not indicate support for the effectiveness of the SOTEP program in reducing sexual and violent recidivism among sex offenders, they do suggest ways in which such a program may be improved. Future research should continue to use controlled investigations on when and how treatment is successful for sexual offenders. Tables, figure, references