One of the key moments at ERS 2021 came in the form of a friendly and informative debate: FeNO* for the diagnosis of asthma in adults: Friend or foe?1 Dr Iñigo Ojanguren Arranz, from the Respiratory Department of Vall d’Hebron University Hospital in Barcelona, outlined the reasons he considers FeNO a friend. Head of Pneumology and Professor of Respiratory Medicine at the University of Liège, Renaud Louis, gave a rebuttal, asserting his thoughts on why FeNO might be a foe.