Rationale: The acidification of the airways seems to be involved in asthma pathophysiology, but its assessment might be difficult. Objectives: The aim of our study is to assess the feasibility and validity of airways acidification measurement by induced sputum and its clinical significance in asthma. Methods: Induced sputum samples were obtained in 57 asthmatic outpatients. The between sample repeatability after 48 hours was measured on an independent population of 14 asthma patients. pH was measured using a pH-meter. The control of asthma was established by the Asthma Control Questionnaire. Measurements: The pH measurement was feasible in all samples and repeatable both within (ICC, intraclass correlation coefficient 0.96) and between sample (ICC 0.621). The mean pH was significantly different between healthy subjects (mean pH 7.54) and asthmatics, both with controlled (7.54 vs 7.28; p=0.0105) and uncontrolled disease (7.54 vs 7.06; p<0.0001) and between stable and poorly controlled asthmatics (7.28 vs 7.06; p=0.0134). The validity of the method was assessed with the ROC curves and induced sputum lower pH values (with a cut-off value of 7,3; sensitivity 72.1%, specificity 100%). Main results and Conclusions: Patients with asthma show lower pH than healthy subjects. Poorly controlled asthmatic patients seem to have the lowest induced sputum pH, independently from the GINA severity level. In conclusion, induced sputum is a feasible, repeatable, non-invasive method to measure airways pH. The pH in induced sputum may reflect a different aspect of asthma from sputum eosinophils and be related to different pathophysiological factors.