Patients with panic disorder are at increased cardiac risk. While the mechanisms responsible remain unknown, activation of the sympathetic nervous system may be implicated. Using isotope dilution methodology, investigations of whole-body and regional sympathetic nervous activity have failed to show any differences between patients with panic disorder and healthy subjects. Using direct recording of single unit efferent sympathetic vasoconstrictor nerve activity by microneurography we examined sympathetic nervous function in patients with panic disorder more precisely than previously reported. The activity of multi unit and single unit vasoconstrictor sympathetic nerves were recorded at rest at the level of the peroneal nerve in 10 patients diagnosed with panic disorder and in 9 matched healthy volunteers. Multiunit sympathetic activity was not different between the 2 groups (26±3 bursts per min in patients with panic disorder and 28±3 bursts per min in controls). The firing frequency of single unit vasoconstrictor neurons was also similar between the 2 groups (0.38±0.09 vs 0.22±0.03 Hz). However, the probability of firing during a sympathetic burst was higher in patients with panic disorder compared with healthy controls (45±5 % vs 32±3%, p<0.05). When only the neural bursts during which the vasoconstrictor neuron was active were considered, we found that in patients with panic disorder the neurons tended to fire more often in a "multiple spike" pattern than in the controls (i.e. the probability of the neuron to fire twice was 25±3% in patients with panic disorder compared with 14±3 % in controls). Quantification from single vasoconstrictor unit recording provides evidence of a disturbed sympathetic firing pattern in patients with panic disorder.