The present study investigated the impact of comorbidity over and above the impact of symptom severity on treatment outcome of Cognitive Behavioral Therapy for children with anxiety disorders. Children (aged 8-12, n = 124) diagnosed with an anxiety disorder were treated with a short-term CBT protocol. Severity was assessed with a composite measure of parent-reported behavior problems. Two approaches to comorbidity were examined; "total comorbidity" which differentiated anxiety disordered children with (n = 69) or without (n = 55) a co-occurring disorder and "non-anxiety comorbidity" which differentiated anxious children with (n = 22) or without a non-anxiety comorbid disorder (n = 102). Treatment outcome was assessed in terms of Recovery, represented by post-treatment diagnostic status, and Reliable Change, a score reflecting changes in pre- to post-treatment symptom levels. Severity contributed to the prediction of (no) Recovery and (more) Reliable Change in parent-reported internalizing and externalizing symptoms and self-reported depressive symptoms. Total and non-anxiety comorbidity added to the prediction of diagnostic recovery. Non-anxiety comorbidity added to the prediction of Reliable Change in parent reported measures by acting as a suppressor variable. Non-anxiety comorbidity operated as a strong predictor that explained all of the variance associated with severity for self-reported depressive symptoms. The results support the need for further research on mechanisms by which treatment gains in children with higher symptom severity and non-anxiety comorbidity can be achieved.