For more than 20 years, the abnormally thick mucus (mucoviscidosis) in cystic fibrosis has been widely shown to be linked to a genetic defect in the cystic fibrosis transmembrane conductance regulator Cl⁻ channel. The defect is widely thought to cause mucus to become dehydrated as a result of basic defects in Cl⁻ dependent fluid transport. However, this widely held explanation is inconsistent with the known physiological properties and functions of organs affected by cystic fibrosis. During the process of releasing highly condensed mucins from intracellular granules, Ca²⁺ and H⁺ cations must be removed to enable the mucins to expand by as much as 1000 times, forming extracellular mucus-gel networks. Over the past few years, that HCO₃⁻ transport is also defective in patients with cystic fibrosis has become apparent. I propose that HCO₃⁻ is crucial to normal mucin expansion because it forms complexes with these cations. Thus, because HCO₃⁻ secretion is defective in cystic fibrosis, mucins in organs affected by cystic fibrosis tend to remain aggregated, poorly solubilised, and less transportable. If the hypothesis is valid, pathogenesis in cystic fibrosis could be due as much to defective transport of HCO₃⁻ as to defective Cl⁻ transport.