Background: Social avoidance and inhibition in animals is associated with hyperresponsiveness of the glucocorticoid stress-system. In humans, the relation between glucocorticoid stress-reactivity and social avoidance behavior remains largely unexplored. We investigated whether increased cortisol stress-responsiveness is linked to increased social avoidance behavior in patients with social anxiety disorder (SAD). Methods: Patients with SAD (n = 18) as well as two control groups of healthy participants (n = 22) and patients with posttraumatic stress disorder (PTSD; n = 17), respectively, performed a social approach-avoidance task (AA-task) in a baseline condition and in a social stress condition (provided by the Trier Social Stress Test). The AA-task is a computerized reaction-time task measuring the speed of manual approach and avoidance responses to visually presented social threat cues (angry faces). Salivary cortisol, blood pressure, and subjective anxiety were assessed throughout the experiment. Results: Patients with SAD showed larger cortisol responses to the social stress test, as compared with healthy and PTSD control subjects. Most crucially, these increased cortisol responses were significantly correlated to the increase in social avoidance behavior measured by the AA-task in the social stress condition in SAD. An additional regression analysis showed that the cortisol responses predicted the stress-induced increase in social avoidance tendencies over and above the effects of blood pressure and subjective anxiety. Conclusions: These findings provide the first evidence for a direct link between increased cortisol stress-responsiveness and social avoidance behavior in patients with SAD. The results support animal models of social avoidance and inhibition and might have important treatment implications.