['It has been observed previously that the pulmonary metastases of colorectal adenocarcinoma are less responsive to therapy with fluorouracil (FUra) as compared with other sites of metastasis (liver, local). To investigate the basis of this chemoresistance, the levels of thymidylate synthase (TS) mRNA and protein were measured, as TS expression has been shown to be predictive of response to therapy in colorectal cancer.', 'Tumors were obtained from 19 patients with metastatic colorectal cancer (12 hepatic and seven pulmonary). TS expression was measured by quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) and TS protein levels were measured by Western blotting. The presence of TS amplification was assessed by Southern blotting. Levels of p53 protein were determined using immunohistochemistry.', 'TS mRNA expression was shown to be significantly higher in the pulmonary metastases (mean TS/beta-actin ratio, 19.7; n = 7) as compared with the hepatic metastases (mean TS/beta-actin ratio, 4.7; n = 11) of colorectal cancer. Lower TS expression was observed in patients with hepatic metastases who had received prior FUra versus patients who had not been treated. High levels of TS expression in some samples was associated with low-level (two to three gene copies) increases in TS gene copy numbers and this was observed more frequently in the pulmonary metastatic samples. The increased gene copy numbers occurred both in samples with wild-type p53 and those with mutant p53 tumor-suppressor gene as determined by immunohistochemistry.'] Are higher levels of thymidylate synthase gene expression observed in pulmonary as compared with hepatic metastases of colorectal adenocarcinoma?