['Thymidine phosphorylase (TP) is an enzyme which converts thymidine to thymine. TP is expressed in a variety of human carcinomas and is known to be a potent angiogenic factor. A recent in vitro study indicated that TP is involved in the intracellular apoptotic signal transduction pathway. The aim of this study was to investigate the correlations between the expression of TP, microvessel density (MVD) and the occurrence of spontaneous apoptosis in esophageal squamous cell carcinoma (ESCC).', 'The expression of TP, intratumoral MVDs and percentages of apoptotic cancer cells, expressed by the apoptotic index (AI), of 155 tumors from 155 patients with ESCC were analyzed by immunohistochemistry and compared.', 'Positive TP expression in cancer and stromal cells was detected in 89 (57.4%) and 104 (67.1%) cases, respectively. The mean MVD and mean AI of the 155 tumors were 288/mm(2) (range: 36-668/mm(2)) and 2.1% (range: 0-20.4%). The mean MVD of 104 tumors with TP-positive stromal cells (336/mm(2)) was higher than that of 51 tumors with TP-negative stromal cells (188/mm(2), p < 0.001). However, the mean MVD of 89 tumors with TP-positive cancer cells (293/mm(2)) did not differ from that of 66 tumors with TP-negative cancer cells (280/mm(2), p = 0.509). On the other hand, the mean AI of 89 tumors with TP-positive cancer cells (1.2%) was lower than that of 66 tumors with TP-negative cancer cells (3.4%, p < 0.001). However, the mean AI of 104 tumors with TP-positive stromal cells (1.9%) did not differ from that of 51 tumors with TP-negative stromal cells (2.6%, p = 0.058). No significant correlation between the MVDs and the AIs was observed (rho = -0.067, p = 0.409).'] Does the expression of thymidine phosphorylase suppress spontaneous apoptosis of cancer cells in esophageal squamous cell carcinoma?