Nocturnal panic (NP), or waking from sleep in a state of panic, occurs in 18% to 45% of panic disorder patients. This relatively common phenomenon, however, is not well understood. In this study, the authors tested the hypotheses that NP represents a more severe form of panic disorder or is a manifestation of heightened vulnerability to sleep disturbance. Patients with NP (N = 51) were compared with patients with panic disorder without a history of NP (N = 41) on measures of panic disorder severity, comorbidity, interpersonal functioning, and sleep disturbance. There was no evidence for more severe psychopathology and only weak evidence for more sleep disturbance. Instead, patients with NP showed less agoraphobic avoidance, perhaps suggesting that they are less likely to associate panic with situational factors. The authors conclude that NP may be a specific version of panic disorder characterized by fearful associations with sleep and sleeplike states.