DNA ploidy microspectrophotometrically determined in 254 patients with gastric carcinoma was investigated from the standpoint of tumor invasion, lymph node metastasis, and prognosis. DNA distribution patterns were grouped into low and high ploidies. The 24.0% frequency in the high ploidy group, at the mucosal stage, increased in proportion to invasion into the deeper layers. There was a high incidence of lymph node metastasis in the high ploidy group, compared with the low ploidy group, in case of invasion beyond the mucosa. Widespread nodal involvement was frequent in the high ploidy group. The 5-year survival rate was 73.7% in patients of high ploidy, with a statistical difference compared to the 90.6% in those of low ploidy (P less than 0.01). In the multivariate analysis of 226 patients undergoing curative surgery, the DNA ploidy proved to be a major independent prognostic factor. These findings indicate a close correlation among DNA ploidy, tumor invasion and nodal involvement, and the significant clinical value of DNA analysis for predicting the prognosis in patients with gastric carcinoma.