1-(Tetrahydro-2-furanyl)-5-fluorouracil, which is named Ftorafur or FT-207 or Tegafur (FT) is used clinically as an antitumor agent. This optically inactive compound was resolved into optically active (R)-(+)- and (S)-(-) isomers. Tegafur is a prodrug of 5-fluorouracil (5-FU) and the bioactivation to 5-FU is mainly catalyzed by cytochrome P450 (CYP) in hepatic microsomes. Although 5-FU was generated from both tegafur enantiomers, R-FT was a preferred substrate than S-FT, because of the considerably higher intrinsic clearance for 5-FU formation from R-FT in liver. The distinct kinetic profiles of the stereoisomers is apparently due to the stereoselective disposition of the R-isomer relative to the S-isomer. These data suggest that the R-isomer of FT is worthy of further preclinical and clinical evaluation for safety, efficacy, and pharmacokinetics.