Thymidine phosphorylase (dThdPase) is an angiogenic enzyme and seems to be related to an angiogenesis in human colorectal carcinoma. The incidence of dThdPase-positive cells was significantly correlated with microvessel count in 21 human colorectal carcinomas. Interleukin 1 alpha (IL-1 alpha), tumor necrosis factor alpha (TNF-alpha), interferon alpha (IFN-alpha) interferon gamma (IFN-gamma) induce dThdPase activity in human cancer cell lines. To study whether this phenomenon occurs in the human colorectal carcinomas, we examined the correlation between dThdPase activity and the expression levels of IL-1 alpha, TNF-alpha, IFN-alpha and IFN-gamma in colorectal carcinoma tissues. dThdPase activity was assayed by the methods of Friedkin and Robert, and the expression level of IL-1 alpha, TNF-alpha, IFN-alpha and IFN-gamma was determined by ELISA. dThdPase activity was significantly correlated with the amount of IL-1 alpha (n = 19, r = 0.347, P = 0.0001), INF-alpha (n = 18, r = 0.717, P = 0.008), and IFN-gamma (n = 4, r = 0.9777, P = 0.0234) in human colorectal carcinomas. However, the dThdPase activity was not correlated with the amount of TNF-alpha (n = 21, r = 0.235, P = 0.2682). These results suggested that the expression levels of IL-1 alpha, IFN-alpha and IFN-gamma are correlated with dThdPase activity in human colorectal carcinomas and that these cytokines may cause angiogenesis by inducing the expression of dThdPase.