Barrett's esophagus (BE) is a specialized columnar epithelium (SCE) that develops as replacement for damaged squamous epithelium (SqE) in subjects with reflux disease, and as such it is apparently more acid resistant than SqE. How SCE resists acid injury is poorly understood; but one means may involve altered tight junctions (TJs) since the TJ in SqE is an early target of attack and damage by acid in reflux disease. To assess this possibility, quantitative RT-PCR for 21 claudins was performed on endoscopic biopsies on SCE of BE and from healthy SqE from subjects without esophageal disease. In SCE, Cldn-18 was the most highly expressed at the mRNA level and this finding paralleled by marked elevation in protein expression on immunoblots. In contrast in SqE, Cldn-18 was minimally expressed at the mRNA level and undetectable at the protein level. Immunofluorescence studies showed membrane localization of Cldn-18 and co-localization with the TJ protein, ZO-1. When Cldn-18 was overexpressed in MDCK II cells and mounted as monolayers in Ussing chambers, it raised electrical resistance and, as shown by lower dilution potentials to a NaCl gradient and lower diffusion potentials to acidic gradients, selectively reduced paracellular permeability to both Na+ and H+ compared to parental MDCK cells. We conclude that Cldn-18 is the dominant claudin in the TJ of SCE and propose that the change from a Cldn-18 deficient TJ in SqE to a Cldn-18 rich TJ in SCE contributes to the greater acid resistance of BE.