The endoscopic records of 56 patients with primary gastric lymphoma and 120 patients with gastric adenocarcinoma (matched for age and sex) were compared with respect to site, extent, and type of tumor by endoscopy and biopsy results. Diagnosis of lymphoma was suspected on the basis of gross endoscopic appearance in 30 patients with 96% sensitivity. The endoscopic findings that achieved statistical significance in favor of lymphoma were extensive disease involving whole stomach, proximal stomach involvement, extension of tumor into duodenum, and the presence of volcano crater-like ulcers on polypoid lesions. Endoscopic biopsies were positive for lymphoma in 92% patients. Immunoperoxidase staining for leucocyte common antigen was positive in all the 24 patients with lymphoma where it was performed. We conclude that lymphoid origin of the stomach tumor may be suspected in one-half of the cases of primary gastric lymphoma by gross endoscopic findings. Multiple endoscopic biopsies are safe and highly sensitive and specific in the diagnosis of primary gastric lymphoma.