OBJECTIVES: In some patients with gastroesophageal reflux disease (GERD), the reflux-damaged esophageal squamous epithelium heals through the process of intestinal metaplasia (resulting in Barrett's esophagus) rather than through the regeneration of more squamous cells. We hypothesized that squamous epithelium in Barrett's esophagus might have abnormalities in activation of the extracellular-regulated kinases 1 and 2 (ERK1/2) signaling pathway that may facilitate esophageal repair through metaplasia in response to acid-induced injury. METHODS: Endoscopic biopsies were taken from distal esophageal squamous mucosa in patients who had GERD with and without Barrett's esophagus and in controls, before and after esophageal perfusion with 0.1 N HCl acid. Basal ERK1/2 phosphorylation, acid-induced ERK1/2 activity and phosphorylation, and localization of phosphorylated ERK1/2 were determined using immunoblotting, Western blotting, and immunohistochemistry. RESULTS: Compared to patients with Barrett's esophagus, patients with GERD exhibited significantly lower baseline levels of phosphorylated ERK1/2 expression (35 +/- 4%vs 90 +/- 21% control, p= 0.01) Acid exposure significantly increased ERK1/2 activity (346.6 +/- 51.90 to 446.8 +/- 62.44 RIU, p= 0.02) and phosphorylation (3.55 +/- 1.26 to 4.49 +/- 1.25 [ratio phospho/total ERK], p= 0.01) in the squamous mucosa of GERD patients, but not in those with Barrett's esophagus or in controls. CONCLUSIONS: Between patients with Barrett's esophagus and patients with uncomplicated GERD, there are significant differences in baseline levels and in acid-induced activation of ERK1/2 in esophageal squamous epithelium. To our knowledge, this is the first description of a molecular, phenotypic feature that distinguishes the esophageal squamous mucosa of GERD patients with and without Barrett's esophagus.