A considerable number of children and adolescents with social anxiety disorder (SAD) do not benefit from treatment as much as expected. However, treatment success should not be measured with social anxiety reports alone; the cognitive, behavioral, and physiological components of social stress should also be assessed. The authors examined an exposure-based SAD-specific group cognitive behavioral therapy (CBT) in a randomized controlled trial (N = 67, age 9-13 years, blind randomized allocation to treatment [CBT; n = 31] and waitlist control [WLC; n = 36] groups). Success was operationalized as a clinically significant reduction of symptoms measured with SAD-specific questionnaires, structured interviews, and changes in response to the Trier Social Stress Test (TSST). In the CBT group, there was a trend toward a significant increase in positive cognitions in the TSST after treatment (d = 0.37), whereas these positive cognitions decreased in the WLC group (d = 0.40). No significant results involving group appeared for negative cognitions, behavior and physiology. Children in the CBT group, but not parents, further reported less social anxiety in one questionnaire from pre-to post-treatment (d = 0.89). A structured interview confirmed a decrease in severity of SAD in the CBT group. While the gold standard of a blind interview showed efficacy of treatment, not all trait and state measures demonstrated similar success patterns. Trial registration Eligibility criteria and some of the dependent variables (cognitions, physiology) for treatment success were registered with the German Research Foundation (TU 78/5-2, HE 3342/4-2) prior to recruitment. Clinical assessment of diagnosis and behavioral data were not a priori planned as outcome measures for this trial and therefore analyzed in a post-hoc approach.