To determine the clinical features and outcome of patients with hypercapnia from acute asthma, we examined 229 (62 men, 167 women) consecutive episodes of acute asthma over a 6-yr period. Sixty-one episodes were associated with hypercapnia at presentation (PaCO2 greater than 38 mm Hg). Men more commonly presented with hypercapnia: 31 of 62 (50%) men with acute asthma had hypercapnia compared with only 30 of 167 (18%) women (p less than 0.001). Patients with hypercapnia had a longer duration of chronic asthma and were more likely to be steroid-dependent. Hypercapnic patients had greater airway obstruction, respiratory rate, and pulsus paradoxus than did nonhypercapnic patients. Findings of a quiet chest on auscultation, inability to talk, and cyanosis also suggested the presence of hypercapnia. Five hypercapnic patients required mechanical ventilation, but hypercapnia did not prolong hospitalization. In nonventilated patients, hypercapnia resolved in a mean time of 5.9 h; 50% of hypercapnic episodes resolved by 4 h, and all resolved by 16 h. No patient presenting with normocapnia progressed to hypercapnia with therapy, and there were no deaths in either the hypercapnic group or the nonhypercapnic group. In patients with more than one admission, the PaCO2 of one episode correlated with the PaCO2 on a subsequent admission, suggesting a role for individual variation in ventilatory control. With appropriate medical therapy, most patients with hypercapnia from acute asthma have rapid reversibility, and mechanical ventilation usually can be avoided. However, these patients require close inhospital observation until it is certain that the acute asthmatic episode has resolved.