Thirty-three patients presenting with dyspepsia were examined with the Olympus Fiber-gastroscope. An endoscopic diagnosis was made in each case and multiple gastric biopsies were obtained for histological and ultrastructural assessment. The endoscopic, histological and ultrastructural findings were compared with each other. Of 33 patients endoscoped, 29 were found to have endoscopic evidence of various types of gastritis and all of these latter had histological evidence of some form of gastritis. Histological confirmation of specific types of endoscopically diagnosed gastritis, however, was only found in 3/9 cases of chronic atrophic gastritis (CAG), 10/14 cases of chronic (superficial) gastritis (CG) and in none of six cases of acute gastritis (AG), indicating that endoscopic diagnosis of specific types of gastritis is relatively inaccurate. Endoscopic diagnosis should thus be restricted to presence or absence of gastritis, leaving the specific typing to histological assessment of the gastric biopsies. Multiple gastric biopsies should be obtained even though the gastric mucosa appears normal endoscopically, since histological evidence of gastritis was found in three out of four cases with endoscopically normal gastric mucosa. Comparison of histological diagnosis with electron microscopy showed that generally there is good correlation between the severity of the histological changes and the ultrastructural grade of damage as defined in this study. It would appear that E.M. examination of the gastric biopsies will not significantly increase the diagnostic accuracy of light microscopy, although it has elucidated the various cellular changes which characterize chronic gastritis. The rough surface contours, the large gastric pit and the increasing number of surface microvilli, seen by scanning E.M., aid the grading of the disease process. Some of these changes are reflected in observations made by transmission E.M. where in addition the basal intercellular edema of the mucosal layer and the appearance of electron-dense mucosal lining cells are observed. The latter, which are the counterpart of "intestinal metaplasia", possess mucus granules which resemble those of gastric mucosal lining cells or intestinal goblet cells, or both.