Immunophenotyping in the management of gastric lymphoma. Primary gastric lymphoma is a rare tumor in which surgical resection plays a major role in improving the response rate and reducing the incidence of bleeding and perforation after chemotherapy. In 17 consecutive patients, the diagnosis of gastric lymphoma was made by immunophenotyping snap-frozen endoscopic biopsy specimens. All neoplasms were B-cell lymphomas. Pan B surface marker antigens were present in all patients. Levels of Ki-67, a nuclear marker of tumor proliferation, were greater than 45% in two of the four patients who died after progression of their lymphoma. All patients alive had Ki-67 levels of less than 30%. A lower proliferation index, as measured by Ki-67, appears to be associated with better prognosis. Ten of 11 patients treated by resection prior to chemotherapy had no complications. Immunophenotyping is the key in the differential diagnosis when considering malignant lymphoma with gastric carcinoma and benign conditions such as pseudolymphoma. 