We estimated the clinical significance of the DNA ploidy pattern and S-phase fraction (SPF) with in vivo administration of bromodeoxyuridine (BrdU) and flow cytometry on 61 early gastric cancers. Aneuploid tumors or tumors with high SPF had high incidence of lymph node metastases than diploid tumors or tumors with low SPF, respectively. Thirty cases of human gastric cancers were examined with both in vivo and in vitro labeling of BrdU. In early gastric cancers, DNA ploidy pattern was same in 11 cases out of 13 cases and SPF was correlated with in vivo and in vitro (p less than 0.01). However, in advanced gastric cancers, DNA ploidy pattern was same in only 3 cases out of 17 cases and SPF was not correlated with in vivo and in vitro. The reasons for no correlation with in vivo and in vitro was reduced to the existence of DNA ploidy heterogeneity which was observed 22.2% and 70.6% in early and advanced cancers, respectively. From the results, we considered that DNA ploidy pattern and SPF were useful marker for lymph node metastases and the preoperative in vitro BrdU labeling method was useful for the marker of lymph node metastasis in early gastric cancers.