Two decades of research demonstrate the efficacy of exposure therapy for posttraumatic stress disorder (PTSD). The efficacy of prolonged exposure (PE), a specific exposure therapy program for PTSD that has been disseminated throughout the world, has been established in many controlled studies using different trauma populations. However, a meta-analysis of the effectiveness of PE for PTSD has not been conducted to date. The purpose of the current paper is to estimate the overall efficacy of PE for PTSD relative to adequate controls. We included all published randomized controlled trials of PE vs. control (wait-list or psychological placebo) for the treatment of PTSD in adolescents or adults. Treatments were classified as PE if they included multiple sessions of imaginal and in vivo exposure and were based on the manualized treatment developed by Foa, Rothbaum, Riggs, and Murdock (1991). Thirteen studies with a total sample size of 675 participants met the final inclusion criteria. The primary analyses showed a large effect for PE versus control on both primary (Hedges's g=1.08) and secondary (Hedges's g=0.77) outcome measures. Analyses also revealed medium to large effect sizes for PE at follow-up, both for primary (Hedges's g=0.68) and secondary (Hedges's g=0.41) outcome measures. There was no significant difference between PE and other active treatments (CPT, EMDR, CT, and SIT). Effect sizes were not moderated by time since trauma, publication year, dose, study quality, or type of trauma. The average PE-treated patient fared better than 86% of patients in control conditions at post-treatment on PTSD measures. PE is a highly effective treatment for PTSD, resulting in substantial treatment gains that are maintained over time.