Thymidylate synthase is a target enzyme for chemotherapeutic agents such as 5-fluorouracil and capecitabine, its oral prodrug. The human thymidylate synthase gene promoter is polymorphic, having either double or triple repeats of a 28-bp sequence. It has previously been shown that colorectal cancer patients who are homozygous for the triple tandem repeats (L/L) have significantly higher thymidylate synthase mRNA expression than those homozygous for the double repeat variant (S/S). Capecitabine is converted to 5-fluorouracil by a sequential triple enzyme pathway, with the last step catalyzed by the tumor-associated angiogenic factor thymidine phosphorylase. We have recently shown that individuals with metastatic colorectal cancer treated with 5-fluorouracil have a higher response rate if they are homozygous for the genotype S/S as opposed to S/L or L/L. Our hypothesis is that individuals homozygous for the double repeat variant (S/S) should have a better response to capecitabine than their counterparts with S/L or L/L. In this retrospective pilot study we assessed the thymidylate synthase polymorphic status of 24 patients with metastatic colorectal cancer and determined their response to capecitabine. We found that 75% (3/4) of individuals with the S/S variant responded to capecitabine, compared to 8% (1/12) and 25% (2/8) of those with the S/L and L/L variants, respectively. Our data suggest that genotyping patients for the thymidylate synthase polymorphism would be useful in identifying patients who are more likely to respond to capecitabine treatment for advanced colorectal cancer.