Data on 100 patients who had undergone resection for poorly differentiated adenocarcinoma of the stomach with serosal invasion were examined to assess the prognostic significance of the DNA distribution pattern in relation to the histologic tumor cell aggregation pattern. DNA distribution patterns were classified into low and high ploidies and tumor cell aggregation patterns were classified into free-cell, small nest, and large nest types. The rates of high ploidy in the free-cell, small nest, and large nest types were 23.1%, 48.0%, and 66.7%, respectively, with a significant increase according to the degree of aggregation. The high ploidy group and large nest type had a higher incidence of lymphogenous and hematogenous metastasis than the low ploidy group and free-cell type, respectively. DNA ploidy had no prognostic value for the free-cell type of tumor, but differences in prognosis and the incidence of metastasis between the DNA ploidies were evident in the nest-forming type. Aneuploid tumors consisting of a poorly differentiated adenocarcinoma were those with nest formation, from the standpoint of histologic structure and metastatic behavior. The DNA analysis of poorly differentiated adenocarcinomas was of prognostic value, especially in the nest-forming type.