The anti-cancer drug, 5'-deoxy-5-fluorouridine (5'DFUR) is known to have antitumor activity through the conversion to 5FU by thymidine phosphorylase (TP). Recently, TP was demonstrated to be identical to angiogenic molecule platelet-derived endothelial cell growth factor (PD-ECGF) by cDNA cloning and subsequent biochemical analyses. We have examined the relationship between the clinical response of 5'DFUR and TP/PD-ECGF expression determined by immunocytochemical analysis in 24 recurrent breast cancer patients. Of 24, 13 were TP/PD-ECGF positive and 11 were TP/PD-ECGF negative. In 13 TP/PD-ECGF positive patients, 4 showed objective response (OR) and 3 showed stable disease (SD) with 5'DFUR treatment, however only one case showed OR and no case showed SD in 11 TP/PD-ECGF negative patients, suggesting that 5'DFUR was likely to be effective for TP/PD-ECGF positive patients. In another group of recurrent breast cancer patients treated by adriamycin containing regimen, no significant correlation was observed between the response of 5'DFUR and the status of TP/PD-ECGF expression. It was indicated that an angiogenic enzyme TP/PD-ECGF expression might be a predictor of the effect of 5'DFUR treatment in human breast cancer.