Atypical attention to potential social threats, such as emotional faces, may be one of the core mechanisms underlying social anxiety disorder (SAD). Pupil dilation is an index of locus coreuleus-noradrenergic activity, and closely linked to attention. In the present study, pupil dilation was studied in adolescents with SAD (N = 26; 22 Female) before the onset of a 12-week cognitive behavioral treatment, and in healthy controls (N = 23). Stimuli were faces with angry or happy emotional expressions. Contrary to our hypothesis, the SAD group did not show hyper-responsiveness to angry compared to happy faces. Instead, an atypical time course of the pupil dilation response was found, resulting in an attenuated response during late time stages. Larger pupil dilation amplitude to happy faces before treatment was related to worse treatment response. These results contribute significantly to our understanding of the mechanisms underlying adolescent SAD.