5-fluorouracil (5-FU) is an important antineoplastic agent that has proven to be effective in the treatment of colorectal cancer. However, one of the main obstacles to the clinical use of 5-FU is the acquisition of resistance to the drug by cancer cells. In vitro studies have demonstrated that the resistance to 5-FU is correlated with increased activity of thymidylate synthase (TS), whose gene has a E2F binding site in its promoter region. To understand the mechanisms through which cancer cells acquire resistance to 5-FU, human colon cancer-derived cell line DLD-1 and its subcloned cell line DLD-1/5-FU, which has acquired resistance to 5-FU, were compared by assessing their phosphorylation of retinoblastoma protein (pRb) and E2F-1 transcriptional activity. The level of pRb phosphorylation in the DLD-1/5-FU cells was higher than in the parental DLD-1cells. In parallel with the increased phosphorylation of pRb, E2F-1 transcriptional activity, which has been shown to be a result of E2F-1 dissociation from hyperphosphorylated pRb, was increased in the DLD-1/5-FU cells. Examination of the effect of E2F-1 decoy oligodeoxynucleotides (ODNs) on the proliferation of DLD-1/5-FU cells in the presence of 5-FU to confirm the importance of E2F-1 in the mechanisms of the acquisition of 5-FU resistance showed that DLD-1/5-FU cells transfected with E2F-1 decoy ODNs recovered their sensitivity to 5-FU. These results suggested that pRb and E2F-1 play important roles in the acquisition of 5-FU resistance by cancer cells and that cancer therapy targeting transcription factor E2F might be effective.