In the quest to investigate small airway function in a range of lung diseases, multiple-breath washout tests have been applied for their potential to represent the most peripheral air spaces [1, 2]. The aim of reducing the burden on the patient has recently also revived interest in the single-breath washout. A single-breath washout test has now been proposed which involves an inhalation of 5% SF₆, 26.3% He, 21% O₂ and the balance as N₂ in order to obtain a so-called dual gas tracer (DTG) phase III slope [3]. After having been introduced as a practical and promising lung function tool [3] and as an early detection tool in cystic fibrosis lung disease [4], the DTG phase III slope is now advocated as a specific index of acinar function abnormality in children with mild asthma [5]. An editorial in the European Respiratory Journal [6], reflecting upon a DTG reproducibility study in normal subjects and chronic obstructive pulmonary disease (COPD) patients [7], rightly pointed out that the clinical utility of DTG indices will depend on their actual physiological meaning. We provide here a critical appraisal of the physiological meaning of the DTG phase III slope.