BACKGROUND Hereditary angioedema due to C1 inhibitor deficiency (HAE-C1-INH) is characterized by relapsing skin swellings, abdominal pain attacks, and, less frequently, potentially life-threatening laryngeal attacks. OBJECTIVE This study determined the mortality of patients with and without the diagnosis of HAE-C1-INH and analyzed fatal laryngeal attacks. METHODS A cohort of 728 patients from 182 families with HAE-C1-INH was evaluated for death cases by analyzing pedigrees. Detailed information on fatal laryngeal attacks in 36 patients was obtained by questioning relatives and treating physicians. RESULTS Of the 214 patients who had died, 70 asphyxiated during a laryngeal attack. Mortality by asphyxiation was higher in patients with undiagnosed HAE-C1-INH (63 cases) than in patients with diagnosed HAE-C1-INH (7 cases). The lifespan of asphyxiated patients with undiagnosed HAE-C1-INH was on average ∼31 years shorter than patients with undiagnosed HAE-C1-INH who died of other causes. Three phases were distinguished in the fatal laryngeal attacks. Phase 1, the predyspnea phase, lasted on average for 3.7 ± 3.2 hours (range, 0-11 hours). Phase 2, the dyspnea phase, lasted on average for 41 ± 49 minutes (range, 2 minutes to 4 hours). Phase 3, the loss of consciousness phase, lasted on average for 8.9 ± 5.1 minutes (range, 2-20 minutes). CONCLUSIONS The high mortality in patients with undiagnosed HAE-C1-INH underscores the need to identify these patients and diagnose their condition. The analysis of fatal laryngeal attacks gives further insight into their course, thus helping to avoid fatalities in the future.