A prospective light and electron microscopic study of repeated endoscopic gastric biopsy specimens obtained from 10 patients presenting at the first examination for long-standing erosive gastritis was performed. In nearly all specimens from 7 patients, spiral bacteria were found in close contact to the surface epithelial cells and associated with active inflammatory changes. These organisms appeared similar to those recently described as gastric "campylobacter-Like organisms" (GCLO). It is possible that they are related to unusual anaerobic spirochetes. In the specimens obtained after antibiotic-therapy the bacteria were few in number and damaged. At the ultrastructural level, the presence of cell necrosis and autolytic intracytoplasmic vacuoles combined with damaged bacteria may suggest a relationship between GCLO surface colonization and erosive epithelial abnormality. Such colonization does not occur in normal mucosa.