The growth fractions of 101 gastric carcinomas were determined in situ by immunostaining with the monoclonal antibody Ki-67 and the results correlated with the histopathologic findings, bromodeoxyuridine (BrdU) labeling index, and DNA ploidy pattern. DNA ploidy patterns were determined by flow cytometric analysis. The Ki-67 labeling rates are rated from 4.6% to 82% (mean, 22%). A significant correlation was found between Ki-67 labeling rates and BrdU labeling indices. Sixty-seven percent of tumors with lymph node metastases showed Ki-67 labeling rates of less than 22%, whereas 33% of tumors without lymph node metastases showed Ki-67 labeling rates of less than 22%. There was a significant correlation between these two groups. Tumors with vessel invasion more often have higher Ki-67 labeling rates than those without vessel invasion. By the DNA ploidy classification, the mean Ki-67 labeling rates of aneuploid tumors was significantly higher than that of diploid tumors. This method yielded similar results to those obtained by BrdU labeling and flow cytometric study. The measurement of Ki-67 labeling rates may be useful to decide the therapeutic modalities.