This study analyzed the history, clinical characteristics, and acid-base data in relation to the speed of decompensation in 34 patients intubated and mechanically ventilated for severe asthma. Three patterns of decompensation were established according to the delay between the onset of symptoms and endotracheal intubation: Group I, rapid decompensation (less than 3 hours); Group II, gradual development of respiratory failure (9.2 +/- 7.7 days); Group III, acute exacerbation after unstable asthma (4.2 +/- 3.6 days). Patients who developed sudden asphyxia (Group I) showed features distinct from those with a gradual worsening. Sudden asphyxic asthma is more frequent in young men and is characterized by a severe mixed acidosis with extreme hypercapnia (mean PaCO2 = 112.8 +/- 43.9 mm Hg), a higher incidence of respiratory arrest, and silent chest upon admission. Recovery is more rapid, with a shorter duration of mechanical ventilation (33.7 +/- 25.3 h versus 91.4 +/- 64.1 h in Group II). Several arguments suggest that bronchospasm plays the primary role in the pathogenesis of sudden asphyxic asthma.