Cystic fibrosis (CF) is caused by the loss of functional CFTR Cl⁻ channels. However, it is not understood how this defect disrupts salt and liquid movement in the airway or whether it alters the NaCl concentration in the thin liquid film covering the airway surface. Using a new approach, we found that CF airway surface liquid had a higher NaCl concentration than normal. Both CF and non-CF epithelia absorbed salt and liquid; however, expression of CFTR Cl⁻ channels was required for maximal absorption. Thus, loss of CFTR elevates the salt concentration in CF airway surface liquid and in sweat by related mechanisms; the elevated NaCl concentration is due to a block in transcellular Cl⁻ movement. The high NaCl may predispose CF airways to bacterial infections by inhibiting endogenous antibacterial defenses.