Helicobacter pylori (HP) may transform from helical bacillary forms to coccoid forms after several days' in vitro incubation. The authors examined 111 consecutive gastrectomy specimens for the presence of coccoid forms of H pylori. Tissues from 64 stomachs (57.7%) showed colonization by H pylori, including 49 cases (76.6%) of adenocarcinoma, 14 cases (21.9%) of benign peptic ulcer, and 1 case (1.6%) of malignant lymphoma. Of these, coccoid forms of H pylori were identified in 53 cases (82.8%). In hematoxylin-and-eosin-stained sections coccoid forms of H pylori appeared as solid, round, basophilic dotlike structures. Under an electron microscope, coccoid forms of H pylori appeared as U-shaped bacilli, with the ends of the two arms joined by a membranous structure. Ultrastructural findings were identical to those from cultures of H pylori. With anti-Helicobacter antibody, coccoid forms of H pylori were positively stained by immunoperoxidase. Helical bacillary forms of H pylori invariably coexisted with the coccoid forms. By semiquantitative analysis, the number of coccoid forms in adenocarcinoma was significantly (P > .01) greater than that in benign peptic ulcers. This study confirms that H pylori can exist in coccoid forms in the human stomach. Coccoid forms should be distinguished from the pathogenic or nonpathogenic bacterial cocci, fungal spores, and cryptosporidia that may colonize the human stomach.