Eosinophilic airway inflammation is a main feature of unstable asthma in adolescents.

•Asthma stability is not precisely defined and not much is known about its relation to airway inflammation.•Eosinophilic inflammation is a dominant cytological phenotype of inflammation in unstable asthma in adolescents.•Simple and easy to perform indirect tests of bronchial hyperresponsiveness might be helpful to assess asthma stability.•Evaluation of cytological profile of inflammation in unstable asthma is useful in the management of eosinophilic asthma.