The aim of our study was to observe the induction of panic attacks by hyperventilation in a group of panic disorder and social phobia patients (DSM-IV). We randomly selected 26 panic disorder patients, 22 social phobics and 25 normal volunteers. They were drug-free for 1 week. Hyperventilation (30 breaths/min) was induced for 3 min. Anxiety scales were taken before and after the test. 61.5% (n = 16) of panic disorder patients, 22.7% (n = 5) of social phobics and 4.0% (n = 1) of control subjects had a panic attack after hyperventilating (p < 0.01, panic disorder vs. control; p < 0.05, panic disorder vs. social phobia; p = n.s., social phobia vs. control). Both anxiety disorder groups were more sensitive to hyperventilation than normal volunteers. The induction of panic attacks by voluntary hyperventilation may be an easy and useful test for validating the diagnosis in some specific panic disorder patients.