Background: Norton and Robinson [2010. Development and evaluation of the anxiety disorder diagnostic questionnaire. Cognitive Behaviour Therapy, 39(2), 137–149. doi:10.1080/16506070903140430] developed the Anxiety Disorder Diagnostic Questionnaire (ADDQ) as a transdiagnostic assessment of fear and anxiety to address problems in using diagnosis-specific measures as well as limitations with the extant transdiagnostic measures of anxiety. The present study validated a weekly version of the ADDQ, the Anxiety Disorder Diagnostic Questionnaire–Weekly (ADDQ-W) allowing session-by-session transdiagnostic assessment of anxiety. Method: Data were a secondary analysis of 49 treatment-seeking outpatient adults from a previous clinical trial. The ADDQ-W was administered weekly over the course of 12-group therapy sessions. Results: The ADDQ-W was a valid weekly measure and neither scores, F(2, 37) = 2.70, p =.08, nor trajectories of change, F(2, 37) = 0.31, p =.73, differed by primary diagnosis, though power was limited. Rate of ADDQ-W change was predictive of change in both primary diagnosis severity, t = 2.40, p =.02, β = 0.32, and overall severity, t = 3.01, p <.01, β = 0.36, at post-treatment. Conclusions: This study has established initial support for the use of the brief, easily scored, ADDQ-W for repeated assessment over treatment using a diagnostically heterogeneous clinical sample of treatment-seeking individuals.