The histological changes that occur in the squamous epithelium in response to acute acid challenge was examined in healthy controls and proton pump inhibitor-treated gastroesophageal reflux disease (GERD) patients and related to the state of untreated erosive GERD in a saline-controlled, randomized perfusion study. In the basal state a stepwise significant increase in the thickness of the basal cell layer, papillary length, and dilatation of intercellular spaces (DIS) was seen when the three groups were compared. Acid perfusion induced a slight increase in the height of the basal cell layer mainly in healthy volunteers; this layer appears to be reactive to acute acid challenge as well as to acid suppressive therapy. DIS increases promptly in response to acute acid exposure in the healthy epithelium but no changes were seen in the lengths of the papillae or regarding DIS in the GERD patients. A protective effect of luminal nitric oxide on DIS development is suggested.