Determination of the treatment "effect size" typically involves calculations that compare the mean outcome (e.g., reduction of recidivism) of a treated group with that of an untreated group, with the comparison being adjusted for the variability in outcome. This literature review focused on five meta-analyses in the literature that provide a basis for estimating effect sizes with sexual offender treatment. The most satisfactory of the studies is the collaborative study sponsored by the Association for the Treatment of Sexual Abusers (ATSA), which involved a meta-analytic review of 43 studies that encompassed 9,454 sexual offenders (5,078 treated and 4,376 untreated participants). All of the studies met the standard of at least providing an incidental matched untreated group. This meta-analytic review has provided strong support for those who consider treatment to be effective with sexual offenders. Over all the studies, the treated participants had lower recidivism rates than the untreated group for sexual crimes (12.3 percent vs. 16.8 percent) and all crimes (27.9 percent vs. 39.2 percent). When only those treatment programs that met current standards were included, the results were even more convincing (9.9 percent in the treated group for sexual recidivism compared to 17.4 percent in the untreated group). In the studies reviewed, cognitive-behavioral programs, with or without relapse prevention components emerged most consistently with higher effect sizes. In all cases, effect sizes for sexual offenders were comparable to, and in some cases better than, effect sizes for the treatment of the other groups. The authors advise that using a harm reduction index to estimate effect sizes for treatment with sexual offenders would produce more meaningful results. 2 tables and 51 references