The purpose of this study was to evaluate the efficacy of group psychotherapy with incarcerated offenders. Meta-analytic techniques were applied to 26 empirical studies that used a treatment group versus a control group. The outcome measures assessed in this study included institutional adjustment, anger, anxiety, depression, interpersonal relations, locus of control, and self-esteem. The results indicate that positive treatment effects were found for the use of group psychotherapy with incarcerated offenders across all outcomes. Supplemental analyses were also included to identify factors that contribute to the efficacy of group psychotherapy and indicate that the use of homework exercises resulted in significantly improved outcomes. Furthermore, participants mandated to treatment did not negatively influence the efficacy of group psychotherapy. Quantitative research synthesis, or meta-analysis, is a set of statistical procedures that allow empirical research from many sources (e.g., research articles) to be integrated for the purpose of creating generalizations (Glass, 1976). Metaanalytic reviews have been frequently utilized to evaluate the effectiveness of correctional programs and to identify what types of treatment programs work with offenders (e.g., Andrews et al., 1990; Lipsey, 1990; Losel, 1995); however, none of these reviews have focused specifically on the use of group psychotherapy with offenders. Group psychotherapy became increasingly popular as a treatment approach with incarcerated offenders in the 1950s because “group therapy has shown itself to be of real institutional value” (Abrahams & McCorkle, 1946, p. 464), and, currently, approximately 20% of male inmates in state correctional facilities receive group psychotherapy services (Morgan, Winterowd, & Ferrell, 1999). Unfortunately, the increased use of group psychotherapy in the past 50 years has not resulted in a parallel increase in research regarding the efficacy of its use. In fact, only 16% of mental health departments in state correctional facilities conduct group psychotherapy research (Morgan, Winterowd, & Ferrell, 1999). Thus, questions regarding the effectiveness of group psychotherapy with incarcerated offenders remain unanswered.