Biopsy specimens from the gastric antral mucosa of 50 patients with upper gastrointestinal complaints were studied by light and electron microscopy and culture. Of 46 assessable specimens, seven were histologically normal, and 39 showed evidence of gastritis. Twenty-seven of the specimens with evidence of gastritis (69%) contained spiral bacteria, whereas only one of the normal specimens (14%) contained these bacteria (P = .02). Of 17 patients found to have gastric ulcers, 10 (59% [P greater than .10] ) also had spiral bacteria. The bacteria could be seen scattered over the surface of the epithelial cells and just under the layer of mucus but were rarely found inside the epithelial cells. Curved or spiral gramnegative bacilli were isolated from 10 of the specimens on chocolate agar incubated at 37 C under microaerophilic conditions. The bacteria resembled the organism recently named Campylobacter pyloridis by other investigators.