Positron emission tomography (PET) with 2-deoxy-2 [18F] fluoro-D-glucose (18FDG), a structural analog of glucose, allows tumor images to be obtained, based on increased glycolysis in cancer cells. In the present study, in order to confirm the effectiveness of 18FDG-PET tumor images for cancer diagnosis, 18FDG tumor uptake values were measured in human gastric cancer xenografts in nude mice. Firstly, comparison of 18FDG uptake and histological grade in four gastric cancers showed that 18FDG uptake increased with loss of differentiation. Secondly, reduction of 18FDG uptake values after administration of anticancer drug was found to correlate with chemosensitivity to anticancer agent, and the values were reduced prior to suppression of tumor growth. These data suggest that 18FDG-PET tumor images may provide reliable indications for assessment of the histological grade of the tumor and the efficacy of cancer chemotherapy.