Treatment programs for sexual offenders have been implemented in prison settings with the objective of reducing recidivism among released offenders. Literature reviews that have evaluated the efficacy of treatment have not found convincing evidence that institutional treatment reduces recidivism; however, these reviews have been primarily concerned with the possibility of a Type I error in hypothesis testing: specifically, that the null hypothesis might be rejected when it is true, concluding that recidivism among treated offenders has been reduced compared with that of untreated offenders and thus concluding that treatment is effective when it is not. The current study used a series of "what if" analyses and power calculations to explore the sensitivity of statistical hypothesis testing in recidivism studies under a variety of sample sizes, base rates, and treatment effects. The size of treatment effects required for significance at the .05 level at various sample sizes and base rates was calculated, and the sample size required to obtain significance at the .05 level in the "average" recidivism study was estimated. The study then examined the sensitivity of statistical hypothesis testing in three of the most-often-cited recidivism studies of institutional sexual offender treatment. Recidivism studies were found to be insensitive to the effects of treatment; these findings are discussed with respect to the likelihood of a Type II error. Alternative methods of assessing sexual offender treatment efficacy are described and recommended. 4 tables, 2 figures, and 26 references