Despite airways which are thought to be normal at birth, 90% of patients with cystic fibrosis (CF) ultimately die from respiratory complications of the disease. The steps involved in the progressive destruction of the airways, and strategies aimed at limiting these processes, are therefore major areas of research. For both research and clinical purposes, measures of lung involvement should ideally be (1) sensitive enough to detect abnormalities early and directly reflect changes in disease severity, either naturally occurring or in response to interventions; (2) feasible and reproducible in all age groups; and (3) repeatable over time.

With regard to the first of these criteria, there is increasing concern that conventional measures such as spirometry and chest radiography are insufficiently sensitive, particularly at the mild and moderate stages of disease. Use of these investigations has enabled a generation of clinicians to improve pulmonary status significantly, but these clinical improvements themselves serve to highlight the inadequacy of the tools we have available to assess them. In contrast to the situation several decades ago, forced expiratory volume in 1 s (FEV₁) now falls too late and too slowly to be accepted unquestioningly as the gold standard. From bronchoscopic studies there is growing concern that, by the time routine spirometry is abnormal, a self-perpetuating infective and inflammatory process has taken hold within the lung that may be difficult to reverse.1 2 Furthermore, reports of …