The authors studied the prognostic values of DNA ploidy pattern and proliferative activity with in vivo administration of bromodeoxyuridine in human gastric cancers. Fresh specimens surgically removed from 117 patients with gastric cancer were investigated by flow cytometric study using a monoclonal antibody to bromodeoxyuridine. DNA ploidy patterns were classified into four types according to the bivariate BrdUrd/DNA distribution: D1, tumors with single diploid population; D2, tumors which showed mosaic of diploid and aneuploid population; A1, tumors with single aneuploid population; and A2, several aneuploid populations without diploid population. The numbers of cases of each ploidy pattern were as follows: D1, 36 cases (30.8%); D2, 38 cases (32.5%); A1, 15 cases (12.8%); and A2, 27 cases (23.1%). DNA ploidy pattern and S-phase fraction (SPF) showed no relation with clinicopathologic findings, except for type A2. In type A2, lymph node metastasis and lymphatic vessel invasion were observed more often than type D1. The SPF calculated from the bivariate BrdUrd/DNA distribution was higher in aneuploidy (D2, A1, and A2) than in diploidy (D1) (P less than 0.01). Also, A2 exhibited a higher SPF than A1 (P less than 0.01). Furthermore, SPF correlated with DNA index significantly (P less than 0.01). Patients who showed aneuploid tumors, DNA ploidy type A2, or SPF of more than 10% survived 3 years less than those with diploid tumors, DNA ploidy type D1, or SPF of less than 10%, respectively (P less than 0.05). By analyzing with the Cox's proportional hazard's model, it is revealed that DNA ploidy and SPF are one of the independent factors of prognostic significance. The results indicated that the patients with aneuploid tumors or highly proliferative tumors had a poor prognosis and that DNA ploidy pattern and SPF were useful prognostic factors for gastric cancers.