Paraffin-embedded tumor samples from 151 patients with stage I gastric cancer were analyzed by DNA flow cytometry, and 80 patients received an infusion of bromodeoxyuridine (BrdU) to determine S-phase fraction. S-phase fractions of tumors were measured by the immunohistochemical method using anti-BrdU monoclonal antibody. Of the 151 patients, 81 (54%), and 70 (46%) showed diploid and aneuploid patterns. There was no significant association between DNA ploidy and wall invasion, histologic type, or lymphatic invasion. Aneuploid tumors were associated with positive-vessel invasion. When the DNA ploidy and clinicopathological parameters were simultaneously entered into the Cox regression model, DNA ploidy and wall invasion emerged as independent prognostic parameters. Aneuploid tumors had significantly higher values of BrdU labeling indices than diploid ones. These results indicate that the determination of DNA ploidy patterns may be an important prognostic factor in patients with stage I gastric cancer, and may be useful in deciding the therapeutic schedule of patients with gastric cancer.