Expectancy theory posits that anxiety sensitivity may serve as a premorbid risk factor for the development of anxiety pathology (S. Reiss, 1991). The principal aim of the present study was to determine whether anxiety sensitivity acts as a specific vulnerability factor in the pathogenesis of anxiety pathology. A large, nonclinical sample of young adults (N = 1,401) was prospectively followed over a 5-week highly stressful period of time (i.e., military basic training). Anxiety sensitivity was found to predict the development of spontaneous panic attacks after controlling for a history of panic attacks and trait anxiety. Approximately 20% of those scoring in the upper decile on the Anxiety Sensitivity Index (R. A. Peterson & S. Reiss, 1987) experienced a panic attack during the 5-week follow-up period compared with only 6% for the remainder of the sample. Anxiety sensitivity also predicted anxiety symptomatology, functional impairment created by anxiety, and disability. These data provide strong evidence for anxiety sensitivity as a risk factor in the development of panic attacks and other anxiety symptoms.