Angiogenesis has an important role in the growth and metastasis of solid tumors. Several angiogenic factors have been identified, one being platelet-derived endothelial cell growth factor (PD-ECGF), which is identical to thymidine phosphorylase (dThdPase). We investigated the activity of dThdPase in 84 samples of 42 human gastric cancers, by liquid chromatography. The dThdPase activity significantly correlated to the microvessel density assessed by immunostaining to CD-31 antigen (P<0.05). Expression of dThdPase has an important role in the promotion of angiogenesis in human gastric cancer.