To clarify the significance of cdc25B, which plays an important physiologic role in regulation of the G2/M check point, in progression of gastric cancer, 125 samples of paraffin-embedded gastric cancers were investigated by immunohistochemistry. In addition, 3 human gastric cancer cell lines were studied to determine the cellular localization of cdc25B by immunohistochemistry and cell fractionation followed by Western blotting. In the cell lines cdc25B was found to be present in both nuclei and cytoplasm, but predominantly in nuclei. High labeling indices of cdc25B in invasion front of gastric cancer was observed in 31 out of 125 cases (24.8%), linked to an advanced depth of cancer invasion (P=0.02), high rates of lymphatic invasion (P=0.03), and lymph node metastasis (P<0.01). Furthermore, the Kaplan-Meier method demonstrated a poor prognosis for cdc25B high labeling indices cases (P=0.02), although multivariate analysis revealed it not to be an independent factor. In conclusion, it seems likely that cdc25B is located predominantly in nuclei when overexpressed and this has some linkage with progression of gastric cancer.