The enzyme, thymidine phosphorylase (dThdPase) is identical to platelet-derived endothelial cell growth factor (PD-ECGF), which acts as a potent angiogenic factor. The present study immunohistochemically examined the expression of dThdPase in human colorectal mucosa, adenomas and carcinomas, as well as six cultured colorectal carcinoma cell lines, in terms of intratumoral microvessel density (IMVD) and P53 expression. Thymidine phosphorylase was observed in lymphocytes, fibroblasts and macrophages, as well as smooth muscle cells and Schwann cells in the peripheral nerve fibers. The dThdPase-positive stromal cells apparently outnumbered the normal epithelial cells, adenoma and carcinoma cells with dThdPase. Weak but obvious cytoplasmic immunoreactivity was noted in a few normal colonic epithelia, predominantly the upper surface area, while a few adenoma cells showed weak nuclear immunostaining for dThdPase in six (24%) of the 25 colonic adenomas. Expression of dThdPase was noted in 33 (73.3%) of the 45 Dukes A and B, 14 (51.9%) of the 27 Dukes C and 14 (56.0%) of the 25 Dukes D carcinomas. The mean IMVD was 84.0 +/- 26.2 in the 36 dThdPase-negative carcinomas and 97.9 +/- 31.6 in the 61 dThdPase-positive carcinomas, the value being significantly higher in the latter group (P < 0.05). The frequency of dThdPase expression was significantly lower in the P53-negative carcinomas than in the positive carcinomas (P < 0.05). Western blot analysis showed the highest expression of dThdPase in LoVo carrying the wild-type p53 gene, followed by Colo201, Colo320, DLD-11 and WiDr carrying the mutated gene. These results indicate that: (i) the main source of dThdPase is stromal cells, including lymphocytes and macrophages in both colorectal normal and carcinoma tissues; (ii) dThdPase may take part in the induction of intratumoral microvessels, regardless of tumor stage; and (iii) expression might be modulated by not only P53 but also other molecules.