The purpose of the present study was to examine directly the role of thymidine phosphorylase (TP) in the sensitivity of renal cell carcinoma (RCC) to a novel fluoropyrimidine carbamate, capecitabine. TP cDNA-transfected RCC are used in these experiments to provide a basis for improved therapeutic benefit in chemoimmunotherapy. Human RCC line KU2 cells were transfected with pcDNA3.1/zeo(+) with or without human TP cDNA by the lipofectin method. We established a clone transfected with pcDNA3.1/zeo(+)/TP (KU2-TP15) and a clone transfected with pcDNA3.1/zeo(+) as a control (KU2-C1). TP expression levels (mean +/- SD) examined by enzyme-linked immunosorbent assay (ELISA) were 1.3 +/- 0.14 U/mg protein in KU2, 1.6 +/- 0.57 U/mg protein in KU2-C1 and 216 +/- 25.6 U/mg protein in KU2-TP15. Immunohistochemical staining of subcutaneous tumors established in Balb/c nu/nu mice showed that KU2-TP15 was strongly positive for TP expression, whereas KU2 and KU2-C1 were negative. Sensitivities in vitro to 5-fluorouracil (5FU), 5'-deoxy-5-fluorouridine (5'DFUR) and capecitabine in KU2-TP15 were significantly enhanced compared with those in KU2 or KU2-C1. A moderate but statistically significant bystander effect was observed in vitro. KU2-TP15 tumors showed significant increase in the in vivo sensitivities to 5'DFUR and capecitabine as compared with the vehicle alone while KU2-C1 tumors did not. The difference in tumor-free rate in mice bearing KU2-TP15 at 2 months after the cessation of treatment was statistically significant between the capecitabine treatment group and the controls, the 5FU treatment group and the 5'DFUR treatment group. The present study clearly provides direct evidence for the role of TP in mediating the sensitivity of RCC to capecitabine.