Background: Patients with anxiety disorder diagnoses commonly have more than one anxiety diagnosis. While cognitive-behavioural interventions have proven efficacy in treating single anxiety disorder diagnoses, there has been little investigation of their efficacy in treating co-occurring anxiety disorders. Aims: To evaluate the efficacy of a transdiagnostic cognitive-behavioural intervention for treating co-occurring anxiety disorders. Method: An A-B single case study design (N = 6) was used to evaluate the efficacy of a 12 to 13-session modular transdiagnostic cognitive-behavioural intervention for treating co-occurring anxiety disorders across patients with at least two of the following diagnoses: GAD, Social Phobia, Panic Disorder and/or OCD. Results: Five of the six participants completed treatment. At posttreatment assessment the five treatment completers achieved diagnostic and symptomatic change, with three participants being diagnosis free. All participants who completed treatment no longer met criteria for any DSM-IV-TR Axis-I diagnosis at the 3-month follow-up assessment, and demonstrated reliable and clinically-significant improvements in symptoms. Across the participants, statistically significant improvements from pre to postintervention were found on measures of anxiety, depression and general well-being, and all improvements were maintained at 3-month follow-up. Conclusions: Results suggest that transdiagnostic cognitive behavioural interventions can be of benefit to patients with co-occurring anxiety disorders.