Although psychiatric comorbidity is common among patients with anxiety disorders, its impact on treatment outcome remains unclear. The present study used meta-analytic techniques to examine the relationship between diagnostic comorbidity and treatment outcome for patients with anxiety disorders. One hundred forty-eight anxiety-disordered treatment samples (combined N=3534) were examined for post-treatment effects from the PsychINFO database. Samples consisted of those exposed to both active (CBT, dynamic therapy, drug treatment, CBT+drug treatment, mindfulness) and inactive treatments (placebo/attention control, wait-list). All treatments were associated with significant improvement at post-treatment, and active treatments were associated with greater effects than were inactive treatments. However, overall comorbidity was generally unrelated to effect size at post-treatment or at follow-up. A significant negative relationship between overall comorbidity and treatment outcome was found for mixed or "neurotic" anxiety samples when examining associations between comorbidity and specific diagnoses. Conversely, there was a significant positive relationship between overall comorbidity and treatment outcome for panic disorder and/or agoraphobia and PTSD or sexual abuse survivors. These findings suggest that while diagnostic comorbidity may not impact the effects of specific anxiety disorder treatments, it appears to differentially impact outcome for specific anxiety disorder diagnoses.