 TS-1 Jeil Pharmaceutical Co.,Ltd, Seoul, Korea is an oral anticancer drug comprised of tegafur, 5-chloro-2, 4-dihydroxypyridine and potassium oxonate, in a molar ratio of 1:0.4:1. Tegafur is a prodrug that generates 5-fluorouracil 5-FU in blood via metabolism by liver enzyme, and 5-chloro-2, 4-dihydroxypyridine enhance the serum concentration of 5-FU by the competitive inhibition of dihydropyrimidine dehydrogenase, an enzyme responsible for 5-FU catabolism. Potassium oxonate is also a reversible competitive inhibitor of orotate phosphoribosyl transferase, a phosphoenzyme for 5-FU. Diarrhea induced by 5-FU administration is though to be attributable to the phosphorylation of 5-FU by the enzyme in the gastrointestinal tissue. After oral administration of potassium oxonate, the concentration of potassium in the gastrointestinal tissue is high enough to inhibit the enzyme while the concentration in blood and tumor is reported to be either slight or nil. Because of these mechanism, oral TS-1 administration generates a higher concentration of 5-FU than protracted intravenous infusion of 5-FU given in a dose equimolar to the tegafur in S-1, whereas the incidence of adverse events concerning the GI tract does not increase. In a phase II trial of TS-1 as first-line setting in NSCLC, the response rate was 22 and the median survival time was 10.2 months. As expected, the incidence of severe gastrointestinal adverse events was low, and so was few severe hematologic toxicity. Recently 3-weekly TS-1 plus cisplatin showed activity against NSCLC with a response rate of 32.7 and the safety was acceptable. 