Dihydropyrimidine dehydrogenase (DPD) is responsible for degradation of the pyrimidines uracil and thymine and the inactivation of the chemotherapeutic agent 5-fluorouracil. DPD activity is highly variable in cancer populations, and this variation may influence the antitumor efficacy of 5-fluorouracil. However, little is known about the regulation of DPD mRNA expression in any tissues. Using a reverse transcription competitive PCR assay, we quantified DPD mRNA levels in 10 matched colorectal tumors and adjacent normal mucosae and 7 colorectal liver metastases and adjacent normal livers. Lower levels of DPD mRNA expression were observed in colorectal tumor compared with adjacent normal colon mucosa (median, 0.01 versus 0.37 amole/microg total RNA, P = 0.02). DPD mRNA expression was also lower in metastases than adjacent normal liver tissue (median, 0.11 versus 1.17 amole/microg total RNA, P = 0.001). DPD mRNA expression was higher in normal liver than normal colonic mucosa (median, 1.17 versus 0.37 amole/microg total RNA, P = 0.02). A significant relationship was observed between DPD mRNA and catalytic activity (r(s) = 0.66, P<0.001). The tumor:normal ratio for DPD mRNA, protein, and activity was relatively stable in liver (0.25, 0.55, and 0.51, respectively) but varied considerably in colon (0.085, 0.9, and 1.25, respectively), consistent with enhanced translation of DPD transcript in primary colorectal tumor. This suggests that DPD can be regulated at the levels of both transcription and translation.