Chronic hypoventilation is a marker of disease severity in asthma and chronic obstructive pulmonary disease (COPD). The degree to which this predicts severity or objective measures of lung function is variable, and more reliable for COPD than for asthma. Patients with COPD are particularly susceptible to nocturnal hypoventilation, but at-risk patients can be identified by abnormalities in daytime gas exchange. Continuous positive airway pressure and noninvasive ventilation have no demonstrated efficacy in the treatment of chronic hypoventilation solely due to asthma or COPD. There is consensus that early treatment with noninvasive ventilation for acute hypoventilation due to a COPD exacerbation is not only highly effective, but it reduces mortality, the need for endotracheal intubation, and hospital length of stay. There is probable benefit to the use of noninvasive ventilation in acute asthma, but the evidence to support this is far less robust than for COPD.