Thymidine phosphorylase (TP), also known as platelet-derived endothelial-cell growth factor (PD-ECGF), is an enzyme that catalyzes the reversible dephosphorylation of thymidine, deoxyguridine and their analogs. TP also has angiogenic properties, although the precise mechanism by which it promotes angiogenesis is not known. TP expression is elevated in many solid tumors including ductal carcinoma in situ and invasive carcinoma of the breast. This has led to intensive study to ascertain whether TP is a biological marker in breast carcinoma; however, the clinical work has produced conflicting results. Some studies have suggested that TP is angiogenic in breast carcinoma, however, we and others have found that TP has little effect on tumor angiogenesis of invasive breast carcinoma. However, increasingly clinical results suggest that TP could represent an interesting marker that could respond to pyrimidine analogues. Widespread application of TP in prognostic testing would require greater uniformity in scoring techniques and determination of the cut-off levels which could distinguish individuals at high and low risk of cancer recurrence and death.