Recently, the efficacy of combined chemotherapy with 5-fluorouracil and low-dose cisplatin for advanced gastrointestinal cancer has been reported in Japan. However, this method has not become established, because the underlying logic is unclear and the quality of its clinical trials has been unsatisfactory. We suggest the mechanism for this depends not only on the action of CDDP as a modulator of 5-FU, but also on a mutual biochemical modulation in which 5-FU acts as a modulator of CDDP and enhances the effect of CDDP. We expect that numerous phase II studies and the accumulation of reliable data will lead to improvements.