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Diffuse airway mucus obstruction is an important feature of severe and fatal asthma. MUC5AC and MUC5B are the principal gel-forming mucins found in airway mucus. The mucin composition of airway mucus likely affects its functional properties.We quantified the principal airway mucins MUC5AC and MUC5B in the sputum of age-matched children with acute and stable asthma and healthy control subjects by using Western blotting.Sputum samples from 38 children (13 with acute asthma, 15 with stable asthma, 10 control subjects) were obtained. Sputum MUC5AC concentrations were 7.6 μg/mL in control subjects, 22.4 μg/mL in those with stable asthma (P = .17), and 44.7 μg/mL in those with acute asthma (P < .05). MUC5B concentrations showed less variation, with 238.5, 208.4 and 165.9 μg/mL in control subjects, those with stable asthma, and those with acute asthma, respectively. The greater MUC5AC concentration in those with acute asthma resulted in a significantly altered MUC5B:MUC5AC ratio between control subjects and those with acute asthma (P < .05). Significant differences in MUC5B glycoforms were present between the groups, with the low-charge-only glycoform being found uniquely in those with acute asthma.Increased MUC5AC and the presence of a low-charge-only MUC5B glycoform significantly altered mucin composition in children with acute asthma. These changes may be important contributory factors to the airway mucus obstruction observed during acute asthma.