Anxiety disorders are among the most common mental health problems in youth, and faulty interpretation bias has been positively linked to anxiety severity, even within anxiety-disordered youth. Quick, reliable assessment of interpretation bias may be useful in identifying youth with certain types of anxiety or assessing changes on cognitive bias during intervention. This study examined the factor structure, reliability, and validity of the Self-report of Ambiguous Social Situations for Youth (SASSY) scale, a self-report measure developed to assess interpretation bias in youth. Participants (N = 488, age 7-17) met diagnostic criteria for social phobia, generalized anxiety disorder, and/or separation anxiety disorder. An exploratory factor analysis was performed on baseline data from youth participating in a large randomized clinical trial. Exploratory factor analysis yielded two factors (accusation/blame, social rejection). The SASSY full scale and social rejection factor demonstrated adequate internal consistency, convergent validity with social anxiety, and discriminant validity as evidenced by non-significant correlations with measures of non-social anxiety. Further, the SASSY social rejection factor accurately distinguished children and adolescents with social phobia from those with other anxiety disorders, supporting its criterion validity, and revealed sensitivity to changes with treatment. Given the relevance to youth with social phobia, pre- and post-intervention data were examined for youth social phobia to test sensitivity to treatment effects; results suggested that SASSY scores reduced for treatment responders. Findings suggest the potential utility of the SASSY social rejection factor as a quick, reliable, and efficient way of assessing interpretation bias in anxious youth, particularly as related to social concerns, in research and clinical settings. ClinicalTrials.gov Number NCT00052078.