Factor analytic studies of the State-Trait Anxiety Inventory (STAI) have provided support for the concepts of state and trait anxiety. This article reports the factor structure of the STAI (Form X) using 205 panic-disordered patients with and without agoraphobia. Results show that a two-factor oblique solution is the most appropriate, accounting for 41.1 per cent of the variance. Eighteen of the A-State items had salient loadings on factor 1 (state anxiety) and all 20 of the A-Trait items had salient loadings on factor 2 (trait anxiety). This study demonstrates the utility of the STAI in measuring anxiety in clinical populations and further supports the theoretical distinction between state and trait anxiety.