Tumor angiogenesis, a crucial step in tumor growth and progression, is regulated by an increasing number of angiogenic factors. One of those is platelet-derived endothelial cell growth factor, recently shown to bethymidine phosphorylase (TP), which reversibly catalyzes the phosphorylation of thymidine to deoxyribose-1-phosphate and thymine. TP overexpression in tumors has been reported, but the differential expression of this enzyme in the colorectal adenoma-carcinoma sequence has not been examined in detail. In this study we analyzed 16 hyperplastic polyps, 37 solitary tubular and tubulovillous adenomas (ranging from 1 to 7.5cm, median 3.2cm), and 47 cases of colorectal carcinomas arising on the basis of pre-existing adenomas (25 cases were Dukes' A, 10 Dukes' B and 12 Dukes' C). Non-neoplastic colonic mucosa was also examined separately from all the above carcinoma cases. All samples were stained for TP and assessed for vascularity. Normal mucosa, hyperplastic polyps, and all but three adenomas and the adenomatous parts of the invading tumors did not show any epithelial cell positivity, and only occasional macrophages and fibroblasts showed weak cytoplasmic immunoreactivity for TP. Neoplastic cells in the carcinomatous part of the tumors were positive for TP in 18 out of 47 (36%) cases. Both nuclear and cytoplasmic staining was detected but in a few cases only one of these was present. There was a highly significant difference between TP expression in neoplastic epithelial cells in adenomas compared with carcinomas (p=0.0001). The same was true when the immunoreactivity of the stromal cells was compared (p=0.0001). Areas with high angiogenesis such as those at the invading edge of the tumor showed intense epithelial, endothelial and stromal TP immunoreactivity. These results show up-regulation of a major angiogenic pathway in both the tumor epithelium and stromal cells with progression from adenoma to carcinoma, and suggest TP may be a candidate target for therapy.