To assist researchers and clinicians in primary care with assessment and treatment of generalized anxiety disorder (GAD), we developed a simple standardized instrument, similar to the Panic Disorder Severity Scale. Independent evaluators used the six-item Generalized Anxiety Disorder Severity Scale (GADSS) to conduct telephone assessments of 330 patients from four primary care facilities with DSM-IV diagnoses of GAD and/or panic disorder who were participating in a study of the effectiveness of collaborative care treatment. Participants were also evaluated at a 12-month follow-up. Internal consistency, convergent and discriminant validity, and sensitivity to change were determined. The GADSS had high internal consistency, and showed good validity and sensitivity to change. Change in GADSS scores discriminated between two different treatment strategies. GADSS is a simple, efficient way to rate severity over the telephone in patients with established diagnoses of GAD.