Inadvertent positive end-expiratory pressure (PEEP) is a potential cause of lung overdistension and impaired gas exchange in ventilated infants. It can be extremely difficult to diagnose clinically and if unrecognized can be life-threatening. Measurement of lung function can lead to the recognition of inadvertent PEEP, allowing appropriate ventilator adjustment with immediate substantial improvement in clinical state. Lung function measurements can help to optimize ventilation and may improve clinical outcome.