Patients who are critically ill and receiving invasivemechanical ventilation are at increased risk for accumula-tion of secretions in the lower airways. Such accumula-tion of airway mucus can induce atelectasis and contributeto ventilator-associated pneumonia. Preventive airwaycare interventions, including humidification, endotrachealsuctioning, and pharmacologic interventions, are thereforefrequently initiated during invasive ventilation. However,evidence for the efficacy of these interventions is scarce,and the absence of guidelines enhances variation in indica-tions for their use. Currently, the choice and timing of interventions aremainly driven by clinical assessment of mucus viscosity based on a mucus classification scale or preference by thetreating physician. Alternatively, airway mucus proper-ties can be measured through rheology, a more objectiveparameter, which characterizes its biophysical properties(eg, viscoelasticity). Previously, studies reported that rhe-ology of airway secretions may help classify chronic muco-obstructive respiratory diseases and serve as a marker ofdisease progression. In this study, we tested the hypoth-esis that airway mucus viscoelastic properties, as measuredby rheology in patients who are critically ill and receivinginvasive mechanical ventilation, correlates with its clinicalmucus classification score.