Cell nuclear DNA content was determined by flow cytometric analysis in 270 patients with advanced gastric cancer. Aneuploid DNA content was observed in 150 patients (55.6%). The DNA ploidy pattern was the third significantly prognostic factor behind peritoneal dissemination and liver metastasis in Cox regression multivariate analysis. About the relationship between DNA ploidy pattern and other prognostic factors, peritoneal dissemination and wall invasion ratio of aneuploid were significantly higher than those of diploid (p less than 0.01). Five-year survival rate of diploidy patients was significantly higher than that of aneuploidy patients. In stage I, five-year survival rate of patients with diploid tumor was 83.3% and that of patients with aneuploid tumor was 70.0%. In stage II, that of patients with diploid tumor was 81.3% and that of patients with aneuploid tumor was 66.7%. In stage III, that of patients with diploid tumor was 71.2% and that of patients with aneuploid tumor was 25.1%. In stage IV, that of patients with diploid tumor was 31.6% and that of patients with aneuploid tumor was 2.6%. Furthermore in the curative case, that of patients with diploid tumor was 77.2% and that of patients with aneuploid tumor was 48.2%. Aneuploid case has significantly worse prognosis in curative operation.