Recently, studies have supported the efficacy of treating anxiety disorders utilizing a transdiagnostic, or non-diagnosis-specific, framework (Erickson, D. H. (2003). Group cognitive behavioural therapy for heterogeneous anxiety disorders. Cognitive Behaviour Therapy, 32, 179-186; Garcia, M. S. (2004). Effectiveness of cognitive-behavioural group therapy in patients with anxiety disorders. Psychology in Spain, 8, 89-97; Norton, P. J., Hope, D. A. (2005). Preliminary evaluation of a broad-spectrum cognitive-behavioral group therapy for anxiety. Journal of Behavior Therapy and Experimental Psychiatry, 36, 79-97). Transdiagnostic group treatment packages focus on the common aspects inherent across the anxiety disorders such as behavioral and cognitive avoidance, and faulty cognitive appraisals of threat potential or meaning (Barlow, D. H., Allen, L. B., Choate, M. L. (2004). Toward a unified treatment for emotional disorders. Behavior Therapy, 35, 205-230). Although research supports the overall efficacy of transdiagnostic cognitive behavior therapy (CBT) for anxiety disorders (Norton, P. J., Philipp, L. M. (2008). Transdiagnostic approaches to the treatment of anxiety disorders: A quantitative review. Psychotherapy: Theory, Research, Practice and Training, 45, 214-226), the effect of diagnostically mixed group composition on individual outcomes is less clear. This study investigated the relationship between group composition and treatment outcome within diagnostically heterogeneous groups with the purpose of determining if diagnostic heterogeneity differentially impacted treatment outcome for 84 individuals during a 12-week transdiagnostic cognitive-behavioral group anxiety treatment program (Norton, P. J. (2012a). Group cognitive-behavioral therapy of anxiety: A transdiagnostic treatment manual. New York: Guilford). The diagnostic makeup of the treatment group was examined at the beginning of treatment and at the end of treatment, and the results indicated that the diagnostic makeup of the treatment group had no significant impact on individual treatment outcome. These findings have direct implications for the delivery of transdiagnostic treatments, and are discussed in terms of their global implications for the transdiagnostic approach to the treatment of anxiety disorders.