The authors examine whether the presence of sleep panic attacks identifies a subgroup of panic disorder patients. Subjects (n = 95) were consecutive patients with panic disorder participating in the MGH longitudinal study of panic disorder. Patients were evaluated with structured interviews to establish adult anxiety and affective disorders, and the presence of childhood anxiety disorders. Patients were queried whether they had ever experienced at least one panic attack during sleep. Patients with a history of sleep panic had significantly higher rates of comorbid generalized anxiety disorder (p < 0.01), social phobia (p < 0.03), and major depression (p < 0.005). The trend was toward longer length of illness (p = 0.09) and were more likely to have a history of an anxiety disorder during childhood (p < 0.005). The presence of sleep panic attacks may delineate a subgroup of panic disorder patients with early difficulties with anxiety, and comorbid mood and anxiety disorders as adults.