The relationship between DNA ploidy and tumor invasion in 254 patients with gastric carcinoma was studied, with particular emphasis on histopathologic features. In the differentiated type of adenocarcinoma, there were aneuploid lesions characterized by a relatively high incidence of lymph node metastasis and hematogenous recurrence, even when the invasion was limited to the submucosa. In the undifferentiated type, aneuploid lesions were rarely seen at the early stage, but the frequency remarkably increased with invasion into the deeper layers. Nodal involvement and disseminating metastasis were evident with serosal invasion. Analyzed by a multivariate Cox model, DNA ploidy significantly correlated with prognosis. Thus, DNA ploidy is a major determinant of survival, and the behavior of an aneuploid carcinoma is apparently controlled by individual histologic type.