Objective—Cognitive-behavioral therapy (CBT) is frequently used for various adult anxiety disorders, but there has been no systematic review of randomized placebo-controlled trials. The present study meta-analytically reviewed the efficacy of CBT versus placebo for adult anxiety disorders. Data Sources—We conducted a computerized search of treatment outcome studies of anxiety disorders between the 1st available year and March 1, 2007. Furthermore, we examined reference lists from identified articles and asked international experts to identify eligible studies. Study Selection—We included studies that randomly assigned adult patients meeting DSM-III- R or DSM-IV criteria for an anxiety disorder to either CBT or placebo. Of 1,165 studies that were initially identified, 27 met all inclusion criteria Data Extraction—The two authors independently identified the eligible studies and selected for each study the continuous measures of anxiety severity. Dichotomous measures reflecting treatment response and continuous measures of depression severity were also collected. Data were extracted separately for completer (25 studies for continuous measures and 21 studies for response rates) and intent-to-treat analyses (6 studies for continuous measures and 8 studies for response rates). Data Synthesis—There were no significant differences in attrition rates between CBT and placebo. Random effect models of completer samples yielded a pooled effect size (Hedges’ g) of 0.73 (95% confidence interval, 0.88–1.65) for continuous anxiety severity measures and 0.45 (90% confidence interval, 0.25–0.65) for depressive symptom severity measures. The pooled odds ratio for completer treatment response rates was 4.06 (95% confidence interval, 2.78–5.92). The strongest effect sizes were observed for obsessive-compulsive disorder and acute stress disorder. The advantage of CBT over placebo did not depend on placebo modality, number of sessions, or study year. Conclusions—Our review of randomized placebo-controlled trials indicates that CBT is efficacious for adult anxiety disorders. There is, however, considerable room for improvement. Also, more studies need to include intent-to-treat analyses in the future.