Supercarbia (PCO2 greater than or equal to 150 torr) may result in a number of pathophysiologic conditions in experimental models and in humans. We report the clinical course and outcome after supercarbia secondary to hypoventilation in five children. Supercarbia resulted from severe airway obstruction in four children and central hypoventilation in one. Maximal PCO2 values ranged from 155 to 269 torr (mean 206). The time course to development of maximal PCO2 was between 35 min and 2 days. The pH ranged between 6.76 and 7.10 (mean 6.86). No patient was severely hypoxemic (PaO2 less than or equal to 55 torr) during the supercarbia period. Despite very high levels of PCO2 and low pH, the only pathophysiologic change found was temporary depression of neurologic function manifested by stupor or coma. Long-term follow-up of these patients has shown no serious adverse neurologic or developmental effects.