In vitro and in vivo studies have been carried out in mouse and human tumors to investigate the biochemical and pharmacologic basis for the selectivity of 5-fluorouracil (FUra) action. Combination chemotherapy with FUra and thymidine was performed to determine the therapeutic relevance of 5-fluorouridine triphosphate (FUTP) incorporation into RNA. The results of these studies indicate that modulation of FUra cytotoxicity by deoxythymidine (dThd) did occur but failed to produce any significant therapeutic advantages in patients with advanced colorectal cancer. Modulation of FUra bioactivation via the deoxyribonucleotide pathway by coadministration of high-dose folinic acid resulted in enhanced therapeutic response rate of gastrointestinal tumor patients. This manuscript summarizes the preclinical and clinical findings on the metabolic modulation of FUra activity by dThd and folinic acid.