Intraepithelial eosinophils in esophageal biopsy specimens were noted to be an indicator of prolonged acid reflux. The presence of even a few intraepithelial eosinophils correlated with abnormal acid clearance determined by overnight intraesophageal pH probe study. This new marker also appeared to be an early lesion, as evidence by its presence in children under age 2 yr, and in biopsy specimens from the proximal esophagus where traditional histometric features (basal zone thickening and papillary lengthening) were lacking. Furthermore, when intraepithelial eosinophils were seen in the proximal 75% of the esophagus, they served to identify more severe disease by correlation with greater abnormalities in the pH probe study. Although this new marker is a histologic indication of prolonged acid reflux and may be appreciated in routine endoscopic biopsy specimens in children, it has been observed in patients over 18 yr of age and may be applicable to the adult population.