Klein's (1993: Arch Gen Psychiatry 50:306-317) "false suffocation alarm" theory of spontaneous panic attacks posits that central receptors compare CO2, O2, and lactate levels and trigger panic when an impending "false" state of suffocation is detected. Several investigators have found abnormalities of respiratory physiology in subjects with panic disorder. Twin and family studies have suggested that both panic disorder and tidal volume response to CO2 are inherited. We hypothesized that, if smothering symptoms are a marker for a hypersensitive suffocation detector and if this hypersensitivity is familial, then relatives of panic subjects with smothering symptoms would have higher rates of panic with smothering than relatives of panic subjects without smothering. We conducted a family study involving 104 panic disorder probands and 247 of their interviewed first-degree relatives. Probands and their relatives were interviewed using the Schedule for Affective Disorders and Schizophrenia--Lifetime Version for Anxiety Disorders to determine their panic disorder and smothering symptom status. Relatives of panic probands with smothering symptoms had an almost threefold higher risk for panic and an almost sixfold higher risk for panic with smothering symptoms when compared with relatives of panic probands without smothering. We conclude that panic disorder with smothering symptoms may be a subtype of panic disorder associated with increased familial risk and may be a group of interest to genetic investigators. These findings provide the first empiric evidence from a family study in support of Klein's false suffocation alarm theory of spontaneous panics.