The clinical, endoscopic, histologic, manometric, and esophageal potential difference characteristics of 20 patients with columnar epithelia lining the lower esophagus (Barrett's esophagus) are presented. Endoscopically, two distinct types were identified: a circumferential-type and an island-type Barrett's esophagus. Patients with these types exhibited similarities in mean age, duration of symptoms, mean lower esophageal sphincter pressure, and frequency of gross esophagitis. Only patients with the circumferential lesion, however, had esophageal strictures or esophageal ulcers. Manometric testing revealed a range of lower esophageal sphincter pressures from 3 to 33 mmHg and qualitative motor abnormalities (i.e., aperistalsis, repetitive waves, tertiary waves) in 3 patients. Histologically, the frequency of epithelial types was junctional greater than specialized columnar greater than atrophic fundic epithelium. More importantly, dysplasia was identified in 2 patients with the circumferential lesion and in 1 patient with the island lesion. Potential difference measurements demonstrated that a high potential difference (greater than -25 mV) was highly specific (92%), but only moderately sensitive (70%) for detecting Barrett's esophagus. Based on these findings, we conclude (a) that there are at least two endoscopically distinct types of Barrett's esophagus involving the lower esophagus--a circumferential type and an island type, (b) that both types are associated with chronic gastroesophageal reflux, with the island type being accompanied by less severe epithelial injury than the circumferential type, and (c) that the identification of dysplasia in the two types suggests that both are unstable lesions requiring continued surveillance with endoscopy and biopsy.