The enzyme thymidine phosphorylase (TYMP) has tumor-promoting functions and its expression is often elevated in tumors. TYMP gene expression in tumorous and mucosal tissues was assessed using real-time polymerase chain reaction, in a study of patients with rectal cancer where chemotherapy and radiotherapy were given sequentially. TYMP levels decreased after chemotherapy. For patients given radiotherapy, there was a significant increase in TYMP expression comparing biopsies before and after radiotherapy. The increase was also observed in the mucosa, although it was less pronounced. Cancer treatment alters gene expression in tumor and adjacent mucosa of patients with rectal cancer. Chemotherapy may cause a decrease in TYMP gene expression, whereas radiotherapy, given as adjuvant treatment, causes a significant increase in expression. These results are of importance when interpreting TYMP expression data in rectal cancer and may be of clinical interest as TYMP participates in the activation of capecitabine.