This study was undertaken to correlate oesophageal histology with the results of potential difference measurements in patients with Barrett's syndrome. Combined manometric and potential difference measurements were performed in 10 patients with Barrett's syndrome and in 12 healthy volunteers. In addition patients with Barrett's syndrome underwent repeated endoscopy with multiple biopsies. In contrast to normal persons, patients with Barrett's syndrome exhibited the potential difference transition zone proximal to the lower oesophageal sphincter. However, compared to histologic findings potential difference measurements underestimated the extent of the pathological epithelium. These differences appear to be due to the occurrence of specialized columnar epithelium in patients with Barrett's syndrome which cannot be differentiated from oesophageal squamous epithelium by potential difference measurements. Thus, PD measurements identify the presence of Barrett's syndrome but do not allow to determine the extent of the disease.