Patients with lung cancer who present with respiratory failure should undergo systematic investigation. Pulmonary function tests, such as measurement of forced expiratory volume in 1 s, carbon monoxide diffusing capacity and measurement of arterial oxygen saturation with pulse oximetry, are commonly used. These tests ascertain the type of defect, for example, obstructive or restrictive ventilatory defects, which helps establish the cause, such as an exacerbation of any underlying obstructive airways disease versus interstitial disease. They also indicate the severity of the disease that helps determine the need for further assessment and treatment.