Background: Mucus is known to play a pathogenic role in muco-obstructive lung diseases, but little is known about the determinants of mucus rheology. The purpose of this study is to determine which sputum components influence sputum rheology in patients with muco-obstructive lung diseases.Methods: We performed a cross sectional prospective cohort study. Spontaneous sputum was collected from consecutive patients with muco-obstructive lung diseases. Sputum rheology was assessed using the Rheomuco® rheometer (Rheonova, Grenoble); the elastic modulus G′, viscous modulus G″, and the critical stress threshold σc were recorded. Key quantitative and qualitative biological sputum components were determined by cytology, nucleic acid amplification tests and mass spectrometry.Results: 48 patients were included from January to August 2019. Among them, 10 had asthma, 14 COPD and 24 non-CF bronchiectasis (NCFB). The critical stress threshold σc predicted a sputum eosinophilia superior to 1.25% with 89.19% accuracy (AUC = 0.8762). G′ and G″ are positively correlated with MUC5AC protein concentration ((rho = 0.361; P = .013) and (rho = 0.335; P = .021), respectively). σc was positively correlated with sputum eosinophilia (rho = 0.394; P = .012), MUC5B (rho = 0.552; P < .001) and total protein (rho = 0.490; P < .001) concentrations. G′ and G″ were significantly higher in asthma patients (G’ = 14.49[7.18–25.26]Pa, G’‘ = 3.0[2.16–5.38]Pa) compared to COPD (G’ = 5.01[2.94–6.48]Pa, P = .010; G’‘ = 1.45[1.16–1.94]Pa, P = .006) and to NCFB (G’ = 4.99[1.49–10.49]Pa, P = .003; G’’ = 1.46[0.71–2.47]Pa, P = .002).Conclusion: In muco-obstructive lung diseases, rheology predicts sputum eosinophilia and is correlated with mucin concentrations, regardless of the underlying disease.Clinical trial registration: (registrar, website, and registration number), where applicable NCT04081740.