Lactate infusions produced panic attacks in one of 15 patients with social phobia, four of nine with agoraphobia, and 10 of 20 with panic disorder in a blind study. The proportion of patients with social phobia who panicked in response to lactate was significantly lower than that of patients with agoraphobia or panic disorder. These findings lend validity to the DSM-III classification of anxiety states. They also suggest that the pathophysiology of social phobia differs from that of disorders characterized by spontaneous panic attacks.