The growth fractions in endoscopically biopsied materials from 121 patients with gastric carcinomas were determined by immunohistochemical staining with the monoclonal antibody Ki-67 and the results correlated with the histopathologic findings and clinical outcome. The Ki-67 labeling rates ranged from 4.6 to 52% (mean: 22%). A significant correlation was found between Ki-67 labeling rates of biopsied materials and those of resected specimens. The tumors showing Ki-67 labeling rates of greater than 22% are more likely to have lymph node metastasis, lymphatic invasion or serosal invasion than those with the rates below 22%. In addition, Ki-67 labeling rates were closely associated with prognosis. Tumors with high Ki-67 labeling rates (greater than 22%) were related to poor prognosis, whereas those with low Ki-67 labeling rates were associated with favorable prognosis. The immunohistochemical staining of proliferating cells in endoscopically biopsied specimens of gastric carcinomas using the Ki-67 monoclonal antibody may be useful in assessing prognosis in carcinoma of the stomach.