We confirmed that the enzyme activity of thymidine phosphorylase (TP) can substitute for 5'-deoxy-5-fluorouridine (5'-dFUR) sensitivity in clinical colorectal cancer (CRC) tissues. Moreover, the idenfication of susceptible genes other than TP will be desired for prediction or treatment of 5'-dFUR. We initially established conditions for the MTT assay system in vivo to compare the growth inhibition rate and TP activity in 18 cases of CRC. TP activity levels were concordant with 5'-dFUR sensitivity in the CRCs (p<0.05). In 18 CRC cases, 4 cases of high sensitivity and 3 cases of low sensitivity to 5'-dFUR were compared to analyze expression profiles of 9437 genes on cDNA microarray chips. As a result, a cluster of genes related to the sensitivity were identified and another cluster of genes related to the insensitivity of 5'-DFUR were also listed. We determined that TP expression can precisely predict 5'-dFUR sensitivity in CRCs and found susceptibility related genes.